14. 6S: 

cop  25 


STATE  OP  ILLINOIS 
DEPARTMENT  OF  REGISTRATION  AND  EDUCATION 

DIVISION  OF  THE 

STATE  GEOLOGICAL  SURVEY 

FRANK  W.  DeWOLF,  Chief 

Cooperative  Mining  Series 

BULLETIN  25 

GAS  PURIFICATION  IN  THE  MEDIUM-SIZE 
GAS  PLANTS  OF  ILLINOIS 


BY 

W.  A.  DUNKLEY,  State  Geological  Survey  Division 

and 

C.  E.  BARNES,  Engineering  Experiment  Station 


ILLINOIS  MINING  INVESTIGATIONS 

Prepared   under   a   cooperative   agreement   between   the  Illinois   State  Geological   Survey 

Division,  the  Engineering  Experiment  Station  of  the  University  of  Illinois, 

and  the  U.  S.  Bureau  of  Mines 


[Printed  by  Authority  of  the  State  of  Illinois] 


URBANA,  ILLINOIS 

1920 


ILLINOIS   MINING   INVESTIGATIONS 

Cooperative  Agreement 
GAS  SECTION 

The  difficulty,  due  to  war  conditions,  of  obtaining  adequate  and  re- 
liable delivery  of  eastern  gas-coal  and  of  coke  has  suggested  the  wider  use 
in  gas  manufacture  of  li$w-sulphur  coal  mined  in  the  central  district,  com- 
prising Illinois,  Indiana,  and  western  Kentucky. 

The  needs  of  the  gas  industry,  and  the  desire  of  the  U.  S.  Fuel  Ad- 
ministration to  meet  those  needs,  has  led  to  the  appointment  by  Governor 
Frank  O.  Lowden,  of  a  Technical  Committee  on  Gas,  By-products,  and 
Public  Utilities,  to  act  in  an  advisory  relation.  The  committee  includes 
representatives  of  the  Illinois  Gas  Association,  the  U.  S.  Bureau  of  Mines, 
the  Engineering  Experiment  Station  of  the  University  of  Illinois,  and  the 
State  Geological  Survey  Division  of  the  Department  of  Registration  and 
Education,  State  of  Illinois. 

Previously,  some  studies  of  the  use  of  Illinois  coal  in  retort-gas  manu- 
facture and  in  by-product  coke  ovens,  and  of  the  chemical  and  physical 
properties  of  Illinois  coal,  have  been  conducted  under  the  Illinois  Mining 
Investigations,  cooperative  agreement — a  joint  agency  of  the  U.  S.  Bureau 
of  Mines,  the  University  of  Illinois,  and  the  State  Geological  Survey  Divi- 
sion. The  continuation  and  expansion  of  this  work  has  been  recommended 
by  the  Technical  Committee  and  the  Fuel  Administration.  In  response  a 
Gas  Section  has  been  created,  and  experienced  gas  engineers,  chemists,  and 
other  specialists  have  undertaken  a  program  of  experiment  on  a  commercial 
scale  to  extend  the  use  of  central  district  coal  in  water-gas  generators  and 
in  gas  retorts. 

The  results  of  the  investigations  will  be  published,  and,  in  addition, 
thp  nnprqtnrs  nf  eras  plants  in  the  region  naturally  tributary  to  central  dis- 
used by  the  Technical  Committee,  of  the  progress  from 
ill  be  urged  to  witness  and  participate  in  the  tests  and 
ir  own  plants  new  or  improved  practices  which  will 
fi  the  railroads,  and  assist  the  mines  and  coke  ovens  to 
ited  demapds  due  to  the  war. 

suggestions   regarding  the  gas  experiments   should  be 
ction,  Room  305,  Ceramics  Building,  Urbana,  Illinois. 


3  3051  00006  3994 


STATE  OF  ILLINOIS 

DEPARTMENT  OF  REGISTRATION  AND  EDUCATION 

DIVISION  OF  THE 

STATE  GEOLOGICAL  SURVEY 

FRANK  W.  DeWOLF,  Chief 

Cooperative  Mining  Series 

BULLETIN  25 

GAS  PURIFICATION  IN  THE  MEDIUM-SIZE 
GAS  PLANTS  OF  ILLINOIS 


BY 

W.  A.  DUNKLEY,  State  Geological  Survey  Division 

and 

C.  E.  BARNES,  Engineering  Experiment  Station 


ILLINOIS  MINING  INVESTIGATIONS 

Prepared   under   a    cooperative   agreement   between    the  Illinois    State  Geological    Survey 

Division,  the  Engineering  Experiment  Station  of  the  University  of  Illinois, 

and  the  U.  S.  Bureau  of  Mines 


[Printed  by  Authority  of  the  State  of  Illinois] 


URBANA,  ILLINOIS 

1920 


STATE  OF  ILLINOIS 
DEPARTMENT  OF  REGISTRATION  AND  EDUCATION 

DIVISION  OF  THE 

STATE  GEOLOGICAL  SURVEY 

FRANK  W.  DeWOLF,  Chief 

Committee  of  the  Board  of  Natural  Resources 
and  Conservation 

Francis  W.   Shepardson,  Chairman 

Director  of  Registration  and  Education 

Kendric  C.   Babcock 

Representing  the  President  of  the  Uni- 
versity of  Illinois 

Rollin  D.  Salisbury 
Geologist 


ILLINOIS  PRINTING  CO.,  DANVILLE,  ILL. 

1920 

37159 — 2m 


CONTENTS 


Purpose  and  scope  of  the  investigation 7 

Inspection  of  gas  plants 7 

Nature  of  data  collected 8 

Summary  of  conditions  observed '. 8 

The  purifying  process 11 

Factors  affecting  gas  purification 11 

Formulas  for  gas  purifiers 12 

Design  of  equipment 12 

Load  factor 12 

Sulphur  content  of  gas 14 

Capacity  of  auxiliary  equipment 15 

Purifier  operation 17 

Uniformity  of  load 17 

Temperature  control 18 

Revivification 19 

Outdoor 19 

In  place 19 

Small  percentage  of  air  with  the  gas 20 

Air  blown  through  oxide  in  off-box 20 

Reversal  of  gas  flow  and  rotation  of  boxes 21 

Chemical  control  and  records 22 

Quality  of  oxide  for  gas  purification 25 

Types  of  hydrated  iron  oxides 26 

Tests  of  oxides 26 

Activity  and  capacity  of  oxides 28 

Conditions  found  in  Illinois  plants 28 

Equipment  conditions 29 

Purifying  equipment  in  individual  plants 30 

Summary  of  capacity  and  load  conditions 37 

Effect  on  purifier  capacities  of  the  use  of  Illinois  coals 41 

Observed  relation  of  oxide  volume  to  purifier  capacity 42 

Rearrangement  of  equipment  to  increase  caj  acity 44 

Results  obtained  in  plants  inspected 46 

Causes  of  low  efficiencies 50 

Overload 50 

Tar  in  the  gas 51 

Methods  of  revivification  in  use 55 

Lack  of  tests  and  records 58 

Cost  of  purification 5{) 

Conclusion 62 

Appendices 

A.  The  Steere  formula  for  gas  purifiers 64 

B.  Sample  record  forms  and  computations  for  keeping  account  of  purification 
and  the  performance  of  oxide  batches 65 

C.  Formula  of  Fulweilcr  and  Kunbcrgcr  for  determining  further  usefulness  of 

oxide  batches 68 


TABLES 

PAGE 

1.  Purifier  load  condition  in  medium-size  water-gas  plants  of  Illinois 38 

2.  Purifier  load  conditions  in  mixed-gas  plants  of  Illinois 39 

3.  Purifying  results  obtained  in  water-gas  plants  inspected 48 

4.  Purifying  results  obtained  in  mixed-gas  plants  insp  ectcd 49 

5.  Distribution  of  tar  through  beds  of  oxide  in  tv\o  plants 52 

6.  Tar  extraction  apparatus  in  use  in  various  plants 54 


ILLUSTRATIONS 


FIGURE 


1.  E^ect  of  hydrogen  sulphide  content  of  gas  on  purifier  capacity 13 

2.  Distribution  of  load  in  a  typical  water-gas  plant 16 

3.  Observed  relation  of  oxide  volume  to  hourly  i  urifier  ( a]  acity 43 

4.  Effect  of  tar  upon  hydrogen  sulphide  absorj  tion  by  iron  oxide 51 

5.  Kunberger  apparatus  for  testing  oxides  for  gas  purification 70 


GAS  PURIFICATION  IN  THE  MEDIUM-SIZE  GAS  PLANTS   OF 

ILLINOIS 

By  W.  A.  Dunkley  and  C.  E.  Barnes 

PURPOSE  OF  THE  INVESTIGATION 

For  several  years,  the  Illinois  State  Geological  Survey  Division,  the 
Engineering  Experiment  Station  of  the  University  of  Illinois,  and  the  U.  S. 
Bureau  of  Mines  have  been  carrying  on  a  cooperative  investigation  of  coal 
and  coal-mining  methods  in  the  central  district,  which  includes  the  states 
of  Illinois  and  Indiana,  and  the  western  end  of  Kentucky.  One  division 
of  the  main  investigation  is  the  use  of  coals  of  this  district  in  gas  manufac- 
ture. A  number  of  bulletins  have  been  published  (see  inside  rear  cover), 
dealing  with  various  phases  of  this  subject.  Gas  purification,  the  topic 
discussed  in  the  present  bulletin,  is  a  phase  of  the  subject  which  has  an  im- 
portant bearing  on  the  use  of  central  district  coals  in  gas  manufacture  with 
the  existing  equipment  and  operating  methods. 

One  of  the  chief  problems  confronting  the  gas  operator  in  using  coals 
of  the  central  district  in  place  of  gas  coals  from  Pennsylvania,  West  Vir- 
ginia, and  eastern  Kentucky,  is  the  increased  amount  of  sulphur  which 
must  be  removed  from  the  gas  before  distribution.  This  increase  may  be 
small  or  large,  according  to  the  particular  coal  used,  but  even  the  central 
district  coals  of  lowest  sulphur  content  usually  contain  more  sulphur  and 
yield  more  to  the  unpurified  gas,  whether  coal-gas  or  water-gas,  than  do 
the  best  low-sulphur  gas  coals  from  the  regions  mentioned,  or  the  cokes 
made  from  those  coals. 

Recognizing  this  condition,  it  was  decided  to  make  a  study  of  existing 
purifying  conditions  in  the  gas  plants  in  Illinois,  to  ascertain  to  what  extent 
the  use  of  low-sulphur  central  district  coals  would  overload  the  purifying 
equipment  now  installed,  and  where  changes  in  equipment,  operating 
methods,  or  purifying  materials  might  be  necessary  to  enable  the  various 
plants  to  purify  the  gas  from  central  district  coals  economically,  should 
other  conditions  make  a  more  extended  use  of  these  coals  desirable. 

Inspection  of  Gas  Plants 

The  problem  of  studying  Illinois  gas-purifying  conditions  was 
assigned  to  W.  A.  Dunkley,  gas  engineer  of  the  State  Geological  Survey 
Division,  and  an  inspection  trip  was  made  by  him  to  sixteen  gas  plants.  The 
plants  visited  comprised  nearly  all  of  the  medium-size  plants  of  the  State, 

(7) 


8  Gas  Purification  in  Medium  Size  Gas  Plants 

including  the  suburban  plants  of  Chicago.  The  urban  plants  of  Chicago 
were  not  studied  at  this  time,  since  it  was  felt  that  on  account  of  their 
large  size  and  special  conditions,  they  might  have  problems  which  were  not 
typical  of  individual  plants  in  smaller  cities. 

Nature  of  Data  Collected 

In  visiting  the  various  plants  an  effort  was  made  to  secure  as  detailed 
information  as  possible  in  the  time  available,  regarding  load  factor,  size 
and  arrangement  of  equipment  for  cleaning  and  purifying  the  gas,  gas 
storage  capacity,  fuels  used,  and  purifying  methods  employed.  A  few 
simple  tests  were  made  in  each  plant  to  determine  the  sulphur  content  of 
the  unpurified  gas  and  the  amount  of  tar  entrained  in  the  gas  entering  the 
purifiers.  Samples  of  spent  purifying  material  and  unused  material  were 
collected  wherever  possible,  in  order  that  information  to  be  gained  from 
chemical  analysis  of  these  materials  might  supplement  the  information 
available  from  inspection  of  the  plants  and  conversation  with  the  operat- 
ors. At  all  stages  of  the  inspection,  hearty  cooperation  was  given  by  the 
gas  men  interviewed,  and  much  interest  was  manifested  by  them. 

With  the  opening  of  the  college  year,  1919-1920,  at  the  University  of 
Illinois,  C.  E.  Barnes,  research  graduate  assistant  in  gas  engineering  at  the 
University,  was  assigned  to  assist  Mr.  Dunkley  in  this  study.  Mr.  Barnes 
devoted  most  of  his  time  to  the  analytical  work  involved  in  studying  the 
purifying  materials  collected  during  the  inspection  of  the  various  plants. 

A  summary  of  the  results  of  these  studies  and  a  statement  of  the  con- 
clusions that  seem  warranted  follow: 

SUMMARY 

To  summarize  briefly,  the  following  purifying  conditions  were  found 
to  exist  in  the  plants  inspected : 

1.  Low-sulphur  eastern  gas  coals  were  being  used  in  practically  all 
of  the  coal-gas  plants  inspected.  These  low-sulphur  coals,  together  with 
the  considerable  percentages  of  water-gas  made  in  most  of  the  plants,  gave 
an  average  H2S  content  in  the  gas  entering  the  purifiers  of  only  250  grains 
per  100  cubic  feet. 

2.  Six  of  the  eight  straight  water-gas  plants  inspected  were  using 
low-sulphur  Illinois  or  Indiana  coals  for  generator  fuel.  The  average 
H2S  content  in  the  unpurified  gas  in  the  water-gas  plants  was  140  grains 
per  100  cubic  feet. 

3.  In  spite  of  the  generally  low  sulphur-content  of  the  gas  to  be 
purified  in  all  the  plants,  75  per  cent  of  the  water-gas  plants  and  50  per 


Summary  9 

cent  of  the  mixed-gas  plants  had  maximum  hourly  gas  productions  in 
excess  of  purifier  capacities.  The  computed  overloads  varied  from  11.5  to 
177  per  cent. 

4.  Only  two  water-gas  plants  and  one  mixed-gas  plant  had  average 
hourly  productions  in  excess  of  purifying  capacity. 

5.  Overload  in  most  cases  was  due  to  lack  of  uniformity  of  load  on 
the  purifiers.  This  in  turn  was  due  to  the  sharp  peak  load  and  insufficient 
holder  capacity.  In  some  cases  the  load  could  probably  have  been  materi- 
ally reduced  by  a  little  more  attention  to  the  rate  of  pumping  gas  through 
the  purifiers. 

6.  Tar  in  appreciable  amounts  was  found  in  the  gas  entering  the 
purifiers  in  nearly  all  the  plants  inspected.  The  spent  oxide  from  all  the 
plants  contained  some  tar.  The  average  tar  content  of  spent  oxides  from 
water-gas  plants  was  6.9  per  cent  and  from  mixed-gas  plants  3.6  per  cent. 
Tar  seemed  to  be  chiefly  responsible  for  low  sulphur  absorption  in  some 
cases. 

7.  The  spent  oxides  from  mixed-gas  plants  had  an  average  sulphur 
content  of  37.4  per  cent,  and  those  from  straight  water-gas  plants  had  an 
average  sulphur  content  of  21.7  per  cent.  Overload  and  tar  seemed  to  be 
mainly  responsible  for  these  conditions  in  some  cases.  In  other  cases, 
operating  methods  seemed  to  be  the  cause  of  these  low  absorptions. 

8.  Revivification  in  place  was  practiced  by  most  water-gas  plants  but 
by  few  mixed-gas  plants  at  the  time  of  inspection.  Only  one  plant  revivified 
oxide  in  the  off-box.  Little  trouble  was  reported  in  that  plant  as  a  result 
of  the  practice,  and  the  operating  costs  were  low. 

9.  Though  several  purifying  installations  are  arranged  for  reversible 
gas  flow,  there  seems  \o  be  little  effort  to  realize  the  fullest  advantage 
from  this  arrangement.     The  same  is  true  of  rotation  of  boxes. 

10.  Few  operators  keep  purifying  records  from  which  actual  per- 
formance of  a  particular  batch  of  oxide  or  method  of  operation  can  be 
definitely  determined. 

11.  Few  operators  make  any  systematic  quantitative  tests  on  their 
purifiers  to  determine  performance  of  the  individual  oxide  batches.  In 
several  plants  where  the  necessary  testing  apparatus  is  available,  it  is 
seldom  used. 

12.  It  seems  that  the  greatest  opportunity  for  immediate  improve- 
ment in  purifying  conditions  rests  in  the  establishment  of  a  simple  but 
regular  testing  routine,  together  with  better  purification  records.  The 
analysis  of  fouled  oxides  for  sulphur  and  tar,  even  if  done  by  an  outside 
laboratory,  would,  it  is  believed,  be  worth  while. 


10  Gas  Purification  in  Medium  Size  Gas  Plants 

13.  Total  purification  costs  for  the  year  1919  varied  in  the  plants 
inspected  from  0.5  cents  to  2.25  cents  per  1,000  cubic  feet  of  gas  purified. 
Careful  operation  and  good  facilities  for  the  handling  of  oxide  were 
apparently  responsible  for  low  costs  in  several  cases  where  the  equipment 
was  overloaded.  Different  conditions  prevailing  in  different  plants  pre- 
clude the  possibility  of  drawing  comparisons  as  to  the  effects  of  load,  etc., 
on  costs. 

14.  Few  different  oxides  are  used  in  the  plants  of  the  State.  It  is 
believed  that  more  experimentation  on  the  part  of  gas  companies,  to  find 
materials  best  suited  to  particular  conditions,  would  be  advantageous. 

15.  The  use  of  low-sulphur  Illinois  and  Indiana  coals  as  water-gas 
generator  fuels  is  general  in  the  water-gas  plants  of  the  State.  Though 
the  sulphur  content  of  the  resulting  unpurified  gas  is  in  some  cases  double 
that  of  gas  from  low-sulphur  eastern  cokes,  the  computed  capacities  of  the 
purifiers  is  but  slightly  less  in  the  former  case. 

16.  Low-sulphur  Illinois  coals  in  coal-gas  manufacture  might  de- 
crease computed  purifier  capacities  by  25  per  cent  in  some  cases,  as  com- 
pared with  the  gas  coals  in  use  at  the  time  of  inspection.  This  decrease 
might  be  offset  in  a  measure  by  more  attention  to  selection  of  oxides,  by 
making  the  load  on  the  purifiers  as  uniform  as  possible,  and  by  more 
attention  to  tar  removal  and  purifying  operation. 

17.  In  several  cases  it  appears  that  coals  of  higher  sulphur  content 
could  be  handled  if  existing  equipment  were  rearranged  and  made  more 
flexible  in  operation.  In  a  few  cases  additional  purifying  apparatus  is 
badly  needed. 


The  Purifying  Process  11 

THE  PURIFYING  PROCESS 

The  purification  of  gas,  by  which  in  the  narrower  sense  is  meant  the 
removal  of  the  sulphur  present  in  the  form  of  hydrogen  sulphide  (H2S),  is 
accomplished  by  the  same  process  in  nearly  all  the  gas  plants  of  the  United 
States.  It  was  discovered  about  35  years  ago  that  hydrated  oxide  of  iron 
was  a  much  more  economical  absorbent  for  this  sulphur  compound  than 
slaked  lime  which  had  been  in  use  since  the  beginning  of  the  gas  industry. 

Oxide  of  iron  does  not  remove  sulphur  compounds  other  than  H2S 
present  in  the  gas,  but  since  the  gas  from  most  American  coals  does  not 
usually  contain  any  excessive  amount  of  these  other  sulphur  compounds,  it 
followed  that  purification  with  hydrated  oxide  of  iron  was  adopted  almost 
universally  within  a  few  years  after  its  initial  use  for  this  purpose. 
Hydrated  oxide  of  iron,  when  of  good  quality,  not  only  absorbs  a  large 
amount  of  hydrogen  sulphides,  but  also,  after  sulphiding,  if  exposed  to  the 
action  of  the  oxygen  of  the  air,  undergoes  a  process  of  regeneration 
whereby  iron  oxide  is  again  formed  by  oxidation  of  a  considerable  portion 
of  the  iron  sulphides  present,  free  sulphur  being  liberated.  This  process  is 
usually  .  called  revivification  by  gas  operators.  After  revivification  the 
material  is  again  in  condition  to  be  used  for  purifying  gas.  Alternate 
sulphiding  and  revivification  may  be  carried  on  until,  with  favorable  con- 
ditions, the  material  may  contain  50  to  60  per  cent  of  sulphide.  It  is  then 
usually  incapable  of  absorbing  more  hydrogen  sulphide  on  account  of  the 
clogging  action  of  the  free  sulphur  and  the  formation  of  more  or  less  inert 
iron  compounds  and  is  replaced  by  new  material. 

The  chemical  reactions  involved  in  the  sulphiding  and  regenerating, 
or  revivifying,  processes  are  not  known  with  absolute  certainty.  Various 
chemical  equations  have  been  written  expressing  the  probable  final  reac- 
tions, but  it  is  likely  that  many  secondary  reactions  really  take  place  which 
are  decidedly  more  complex  than  those  given  in  the  text  books.  Since  it  is 
the  intention  to  confine  this  bulletin  to  the  practical  working  phases  of  the 
purifying  process,  no  attempt  will  be  made  here  to  repeat  these  equations 
or  to  go  deeply  into  the  theory  of  the  reactions.  The  reader  is  referred  to 
the  literature  of  gas  manufacture  and  chemistry  for  existing  information 
on  this  subject. 

FACTORS  AFFECTING   PURIFICATION 

Efficiency  and  economy  in  gas  purification  depend  upon  three  main 
factors — equipment,  operating  methods,  and  purifying  material.  The 
effect  of  each  of  these  factors  is  more  or  less  determined  by  existing  plant 
conditions.  Perhaps  the  most  important  condition  affecting  plant  condi- 
tions is  the  load  factor.     Load  factor  as  applied  to  purification  will   here 


12  Gas  Purification  in  Medium  Size  Gas  Plants 

be  designated  as  the  relation  between  the  volume  of  gas  passing  through 
the  purifying  equipment  during  the  hour  of  maximum  production  and  the 
rated  hourly  capacity  of  that  equipment,  as  determined  from  its  dimensions 
and  from  practical  and  theoretical  considerations  pertaining  to  operation 
with  the  usual  purifying  materials. 

Formulas  for  Gas  Purifiers 

Since  the  advent  of  the  gas  industry,  nearly  20  formulas  have  been 
propounded  for  the  dimensioning  of  gas  purifiers.  Many  of  these  were 
based  upon  the  use  of  slaked  lime,  the  predecessor  of  hydrated  oxide  of  iron 
for  purification.  Many  are  indefinite  in  their  terms  and  include  an  insuf- 
ficient number  of  factors  to  make  them  really  applicable  to  present  condi- 
tions. The  formula  of  the  Steere  Engineering  Company1  of  Detroit, 
which  appeared  about  a  year  ago,  is  perhaps  the  most  complete  of  all,  and 
while  there  are  still  factors  which  will  probably  have  to  be  introduced  or 
changed  in  it  when  our  knowledge  of  these  factors  becomes  more  complete, 
the  formula  is  very  useful  and  has  been  used  in  the  computations  of  this 
bulletin.  For  the  convenience  of  readers,  the  Steere  formula  and  some 
information  regarding  its  use  are  given  in  Appendix  A. 

Design  of  Equipment 

In  designing  purifying  equipment  for  a  given  plant,  a  number  of 
things  must  be  considered.  One  of  the  most  important  is  the  output  of 
the  plant,  both  present  and  prospective.  In  spite  of  electric  competition, 
the  output  of  most  gas  plants  is  growing  rapidly;  in  fact,  many  companies 
are  experiencing  difficulty  in  keeping  up  with  the  demand  for  gas.  It  is 
important,  therefore,  to  make  the  purifying  equipment  of  ample  size,  but 
at  the  same  time  the  investment  is  heavy  and  the  interest  on  a  greatly 
oversized  equipment  may  offset  to  a  considerable  extent  the  operating 
advantages  which  might  be  derived  from  extra  large  capacity. 

Load  Factor 

Another  factor  to  be  considered  is  the  distribution  of  load  during  the 
day.  The  rate  of  output  of  most  plants  is  far  from  uniform.  Indeed,  it 
is  not  unusual  for  some  plants  to  put  out  10  per  cent  of  their  daily  produc- 
tion during  the  hour  of  maximum  load.  And  since  storage  capacity  has 
not  usually  grown  apace  with  output,  it  is  frequently  necessary  to  generate 
and  purify  the  gas  practically  as  fast  as  it  is  sent  out.     From  the  figures 


iGas  Age,  Vol.  43,  p.  227,  1919. 


Factors  Affecting  Purification 

maximum  hourly  purifier  capacity— m  cu.  ft. 


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14  Gas  Purification  in  Medium  Size  Gas  Plants 

obtained  from  the  inspection  of  16  gas  plants  of  Illinois,  of  which  8  made 
straight  water-gas  and  8  mixed  coal-  and  water-gas,  it  was  found  that  the 
maximum  volume  of  gas  purified  per  hour  in  water-gas  plants  varied  from 
4.5  per  cent  to  11.2  per  cent  of  the  maximum  daily  output,  with  an  average 
of  7  per  cent;  and  in  mixed-gas  plants  from  3.6  per  cent  to  9.3  per  cent, 
with  an  average  of  about  6  per  cent.  Since  the  complete  absorption  of 
hydrogen  sulphide  from  the  gas  by  iron  oxide  takes  a  measurable  time,  the 
purifiers  must  be  so  designed  that  even  at  maximum  rate  of  flow  there  will 
be  ample  time  of  contact  between  gas  and  oxide,  even  when  the  oxide  is 
partially  sulphided  and  inactive.  As  the  laws  of  nearly  every  state  require 
that  the  gas  leaving  the  gas  plant  must  at  all  times  be  free  from  any  appre- 
ciable amount  of  hydrogen  sulphide,  the  gas  manufacturer  must  comply 
with  this  requirement  by  whatever  means  he  may.  Oftentimes  when  the 
purifying  equipment  is  heavily  overloaded  or  fuels  run  considerably  higher 
in  sulphur  content  that  usual,  compliance  with  the  law  is  difficult  and 
costly. 

As  an  economic  matter,  the  purifiers  must  be  so  designed  that  they 
will  hold  enough  oxide  to  completely  purify  the  gas  at  the  maximum  rate 
of  flow  for  a  sufficient  time  so  that  it  will  not  be  necessary  to  handle  the 
oxide  too  frequently.  It  is  desirable  that  the  oxide  be  allowed  to  remain 
in  the  purifiers  long  enough  between  revivifications  so  that  it  will  absorb 
a  reasonable  amount  of  sulphur.  The  labor  cost  of  handling  oxide  is  one 
of  the  heaviest  items  of  purification  cost,  and  it  is  therefore  desirable  that 
an  oxide  take  up  a  maximum  amount  of  sulphur  with  the  least  cost  of 
handling.  This  can  be  done  by  making  the  purifiers  large  enough,  due 
consideration  being  given  to  investment  charges. 

Sulphur  Content  of  Gas 

The  sulphur  content  of  the  gas  to  be  purified  is  another  factor  to  be 
considered.  The  gas  industry  has  always  been  a  particular  customer  in 
the  purchase  of  coal,  and  the  sulphur  content  has  always  been  an  important 
specification  where  there  was  otherwise  little  choice  between  coals.  Good 
gas  coals  heretofore  have  contained  not  exceeding  one  per  cent  of  total 
sulphur,  and  frequently  the  content  of  sulphur  has  been  only  .5  or  .6  of  one 
per  cent.  The  decrease  in  supply  of  such  superfine  coals  has  caused  gas 
operators  to  look  about  for  possible  new  supplies,  but  while  the  industry 
will  probably  have  to  be  reasonably  particular  so  long  as  present  purifying 
methods  are  in  use,  it  will  probably  be  necessary  to  use  coals  of  higher 
sulphur  content  than  would  heretofore  have  been  considered  expedient.  In 
water-gas  not  only  the  sulphur  content  of  the  generator  fuel,  but  also  the 


Factors  Affecting  Purification  15 

sulphur   in   the   enriching  oil   must   be   considered.      Purifying   equipment 
consequently  will  have  to  be  designed  with  these  matters  in  mind. 

The  maximum  permissible  rate  of  gas  flow  through  a  system  of 
purifiers  does  not  vary  inversely  as  the  hydrogen  sulphide  content  of  the 
gas  to  be  purified.  Figure  1,  which  is  plotted  from  the  Steere  formula, 
shows  the  relation  existing  between  maximum  hourly  rate  of  gas  flow 
through  the  purifiers  and  the  hydrogen  sulphide  content  of  the  gas.  The 
curve  represents  the  maximum  hourly  purifying  capacity  with  various  con- 
tents of  hydrogen  sulphide  of  a  plant  which  would  have  a  capacity  of 
100,000  cubic  feet  of  gas  per  hour,  if  the  gas  to  be  purified  contained  200 
grains  of  hydrogen  sulphide  per  100  cubic  feet.1  It  will  be  noted  that  if 
the  sulphur  content  is  multiplied  by  5,  giving  1,000  grains  per  100  cubic 
feet,  the  capacity  is  reduced  from  100,000  cubic  feet  to  66,500  cubic  feet. 
Capacity  in  this  sense  pertains,  of  course,  to  the  hourly  rate  of  gas  flow 
permissible,  not  to  the  capacity  of  the  oxide  for  absorbing  H2S.  If  the 
oxide  in  the  purifiers  could  be  completely  fouled  in  either  case,  it  is  evident 
that  five  times  as  much  gas  containing  200  grains  of  H2S  could  be  purified. 
It  is  evident,  therefore,  that  the  permissible  rate  of  gas  flow  through  the 
purifiers  is  not  directly  proportional  to  the  absorption  capacity  of  the  oxide 
nor  inversely  proportional  to  the  sulphur  content  of  the  gas.  The  rate 
of  the  chemical  reaction  has  an  important  bearing,  but  this  is  not  taken 
directly  into  consideration  in  any  existing  formula  for  the  design  of  puri- 
fiers, though  it  is  indirectly  allowed  for  in  the  Steere  formula. 
Capacity  of  Auxiliary  Equipment 

While  the  capacity  of  the  oxide  purifiers  must  be  designed  to  handle 
the  maximum  hourly  load,  a  properly  designed  system  may  fail  to  accom- 
plish the  complete  purification  of  the  gas  because  of  conditions  existing  in 
other  units  of  the  gas-cleaning  apparatus.  Under  favorable  operating  con- 
ditions, the  purifiers  are  not  usually  called  upon  to  handle  all  of  the 
hydrogen  sulphide  that  is  originally  present  in  the  gas  when  generated. 
Water  and  tar  vapor  condensing  from  the  gas  in  the  condensers,  and  wash 
water  in  the  scrubbers  all  remove  a  certain  amount  of  ITS  from  the  gas. 
In  coal-gas  plants  the  ammonia  present  in  the  gas  has  a  very  important 
part  in  sulphur  removal,  since  it  combines  directly  with  H2S.  There  is 
not  enough  ammonia  present  to  remove  all  the  sulphur,  but  this  incidental 
purification  may  remove  as  much  as  20  per  cent  to  40  per  cent  of  the  ITS 
present  in  the  gas.     If  the  condensing  and  scrubbing  apparatus  is  under- 


Ut  may  be  obiected  bv  some  that  when  the  iiti9  no  H2S  at  all  the  capacity  of  the  purifici  s 

would  be  infinite  and  that  there  should  be  consenuently  a  shrrp  rise  in  the  curve 

as  sew  n  as  there  is  any  appreciable  amount  of  H2?  present  in  the  (?p«,  the  time  Fa  tor  of  the  reaction 
between  H2S  and  hydrated  oxide  of  iron  comes  into  play,  necessitating  a  very  a  preciable  time  of 
contact  to  purify  the  pas.  It  is  often  observed  in  practice  thai  it  is  more  diffi cull  to  remove  the  lasl  10 
•  t  of  H2F  from  the  gas  than  the  first  90  per  cent,  emphasizi-  g  the  fact  that  the  permissible  rate 
of  purification  is  not  inversely  proportional  to  the  H2?  content  of  the  g?s. 


16  Gas  Purification  in  Medium  Size  Gas  Plants 

gas  purified  per  hour— m.  cu.  ft. 


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Purifier  Operation  17 

sized,  not  only  is  this  incidental  purification  diminished,  but  the  oxide 
purifiers  are  forced  to  do  part  of  the  work,  namely,  tar  extraction,  which 
should  be  confined  to  the  first-mentioned  equipment.  Tar  and  oil  vapors 
undoubtedly  have  a  detrimental  effect  on  the  operation  of  the  purifiers, 
since  they  coat  the  purifying  material  and  render  it  partly  inactive.  De- 
spite the  recognized  harmful  effect  of  these  vapors,  there  are  few  plants 
in  which  the  gas  entering  the  purifiers  is  entirely  free  from  them.  In  some 
plants  well  equipped  as  to  purifier  capacity  this  condition  is  responsible  for 
poor  purifying  results.  Tar  removal  before  purification  is  very  important 
both  as  a  matter  of  equipment  design  and  of  operation,  and  deserves  more 
attention  than  it  usually  gets.  Some  kind  of  test  by  which  the  tar  content 
of  the  gas  entering  the  purifiers  can  be  determined,  should  be  made  regu- 
larly where  purifying  results  indicate  the  possibility  of  this  trouble.  Fre- 
quently a  plant  which  seems  to  need  additional  purifying  capacity  is  really 
in  far  greater  need  of  more  efficient  tar-extracting  apparatus. 

PURIFIER  OPERATION 
Uniformity  of  Load 

Granted  adequate  purifying  equipment,  the  purifying  efficiencies 
realized  will  depend  much  upon  how  the  equipment  is  operated.  First, 
the  handling  of  the  load  may  well  be  considered.  The  hourly  rate  of  gas 
output  from  a  plant  is,  of  course,  out  of  the  control  of  the  gas  manufac- 
turer. He  must  supply  the  demand  as  needed.  For  the  sake  of  safety  to 
tide  over  any  accident  to  the  gas-producing  equipment,  it  is  usually  con- 
sidered necessary  to  keep  the  gas  storage  holders  as  nearly  full  as  possible 
at  all  times.  If  the  storage  capacity  is  much  undersize,  in  order  to  make 
good  the  output,  it  may  be  necessary  to  purify  the  gas  almost  at  the  rate  of 
output  during  certain  hours  of  the  day.  This  may  necessitate  purifying  the 
gas  at  a  rate  much  in  excess  of  the  rated  purifying  capacity  during  such 
times.  Even  so,  it  may  be  possible  by  attention  to  smooth  out  the  produc- 
tion curve  somewhat.  Figure  2  shows  the  output  and  production  rates  in 
a  typical  water-gas  plant  operating  24  hours  per  day.  The  rated  capacity 
of  the  purifiers  and  the  average  hourly  make  are  shown  by  horizontal  lines. 
It  will  be  noted  that  the  hours  of  large  production  do  not  always  coincide 
with  the  hours  of  large  output.  The  production  curve  crosses  and  re- 
crosses  the  line  of  average  production  not  only  when  the  production  is  near 
the  average  for  a  considerable  time,  but  also  when  it  is  averaging  consider- 
ably above  or  below  the  general  average  for  some  time.  By  careful  atten- 
tion to  the  rate  of  pumping  gas  these  wide  fluctuations  could  probably  be 
prevented  to  a  considerable  extent  with  benefit  to  purifying  operation. 


18  Gas  Purification  in  Medium  Size  Gas  Plants 

Temperature  Control 

Temperature  control  is  another  important  consideration  in  purifier 
operation.  Both  excessively  high  and  excessively  low  temperatures  at  the 
purifiers  should  be  avoided.  The  statement  is  often  made  that  at  low 
temperatures  (below  about  60 °F.)  the  sulphiding  reaction  becomes  slug- 
gish. That  there  is  some  difference  of  opinion  relative  to  the  effect  of 
temperature  on  the  purifying  process,  is  indicated  by  the  fact  that  this 
matter  is  now  being  studied  by  the  Purification  Committee  of  the  American 
Gas  Association.  A  temperature  of  about  100°F.  is  usually  thought  to 
give  the  best  results,  and  often  it  is  considered  important  that  the  tempera- 
ture be  kept  up  in  winter  by  artificial  means  if  necessary.  Formerly  it  was 
the  practice  to  have  the  purifiers  installed  indoors,  but  the  high  cost  of 
building  construction  as  well  as  successful  experiments  with  outdoor  puri- 
fiers have  resulted  in  a  rapidly  increasing  number  of  outdoor  installations. 
Practically  all  of  the  new  installations  are  of  the  latter  type.  Where  the 
temperature  of  the  gas  has  a  tendency  to  fall  in  winter  much  below  the 
temperatures  above  stated,  it  can  usually  be  kept  up  by  the  installation  of 
steam  coils  in  the  purifiers  or  by  injection  of  steam  into  the  gas  ahead  of 
the  purifiers.  The  latter  practice  may  be  questioned  by  some  operators  on 
account  of  the  amount  of  moisture  which  is  deposited  in  the  purifying 
material,  while  the  former  may  be  open  to  objection  on  the  ground  that  it 
dries  out  the  oxide  too  much.  Some  outdoor  installations  are  insulated  to 
diminish  the  drop  in  temperature  during  cold  weather,  but  this  is  not  com- 
mon practice,  at  least  in  Illinois. 

While  a  fairly  high  temperature  is  usually  considered  advantageous  in 
its  effect  on  the  sulphiding  reaction,  the  temperature  of  the  gas  throughout 
the  condensing  and  purifying  system  should  not  be  maintained  too  high, 
lest  difficulty  be  experienced  in  extracting  the  tar.  A  temperature  much 
above  100°  at  the  inlet  to  the  purifiers,  if  maintained  by  the  original  heat 
in  the  gas  as  generated,  would  usually  necessitate  a  temperature  consider- 
ably in  excess  of  this  back  at  the  tar-extracting  equipment.  It  is  usually 
considered  that  100°  to  U0°F.  is  about  the  highest  temperature  at  which 
tar  can  be  extracted  efficiently  by  most  forms  of  tar  extractors,  though 
opinion  may  vary  on  this  point.  During  the  inspection  trip  to  various 
Illinois  plants  by  the  writer,  several  cases  were  observed  where  the 
temperature  at  the  inlet  of  the  purifiers  was  around  120°  to  130°,  but  in 
practically  every  such  instance  there  was  an  excessive  amount  of  tar  being 
carried  into  the  purifiers.  Probably  the  best  results  will  be  obtained  by 
maintaining  a  temperature  of  90°  to  100°F.  at  the  boxes  and  keeping  the 
gas  saturated  with  water  by  the  admission  of  steam  to  the  gas,  or  other- 


Purifier  Operation  19 

Revivification 
outdoor  revivification 

As  previously  mentioned,  the  greatest  advantage  of  hydrated  oxide  of 
iron  as  an  absorbent  for  hydrogen  sulphide  lies  in  its  ability  to  revivify  or 
regenerate  when  after  sulphiding  it  is  exposed  to  the  oxygen  of  the  air. 
Naturally  the  first  method  of  revivification  adopted  was  to  remove  the 
sulphided  material  from  the  purifying  box  and  expose  it  to  the  air.  The 
appearance  of  the  material  as  it  changes  from  the  black  iron  sulphide  to 
red  or  brown  iron  oxide,  is  of  course  a  guide  to  the  operator  by  which  he 
can  determine  with  more  or  less  certainty  when  the  material  is  reoxidized 
and  ready  for  use  in  the  purifiers  again.  The  usual  procedure  in  revivify- 
ing out  of  doors  is  to  put  the  material  in  piles  or  windrows,  perhaps  two 
or  three  feet  high.  When  the  material  begins  to  heat  as  a  result  of  the 
oxidation  process,  it  is  raked  down  into  a  layer  a  foot  or  so  in  thickness, 
and  as  the  surface  reddens,  the  whole  mass  is  turned  over  by  shovel,  this 
operation  being  repeated  until  the  mass  is  of  uniform  color  and  no  longer 
heats.  This  process,  simple  as  it  seems,  requires  considerable  attention. 
The  material  frequently  oxidizes  very  rapidly  upon  removal  from  the 
purifiers,  and  when  it  contains  a  considerable  percentage  of  iron  sulphide 
it  is  likely  to  ignite,  especially  if  in  a  deep  mass  from  which  heat  cannot 
escape  readily.  Overheating  is  detrimental  to  the  further  value  of  the 
material,  rendering  it  inactive.  At  the  same  time,  a  moderate  degree  of 
heat  promotes  the  oxidizing  reaction  without  injuring  the  material.  The 
operator  therefore  usually  cools  hot  spots  in  the  material  by  shoveling  them 
out  and  exposing  the  hot  material  to  the  open  air,  which  rapidly  cools  it, 
rather  than  by  application  of  water,  which  cools  the  material  to  such  an 
extent  as  to  unduly  retard  the  revivification. 

This  method  of  revivification  involves  much  handling  of  the 
material.  Indeed,  since  it  is  not  possible  to  leave  material  in  the  boxes 
until  it  is  completely  sulphided,  it  is  usually  necessary  to  handle  it  a  dozen 
times  or  more  to  get  a  very  good  sulphide  content.  But  since  the  sulphid- 
ing and  revivifying  reactions  slow  down  greatly  after  a  time,  the  cost  of 
handling  the  material  may  soon  offset  the  value  of  the  work  it  is  doing. 
Consequently,  material  is  frequently  discarded  long  before  it  contains  50 
to  60  per  cent  sulphur,  which  is  considered  good  operation. 

REVIVIFICATION    IN    PLACE 

Naturally  gas  operators  looked  for  a  method  of  revivifying  which 
involved  less  handling  of  oxide,  and  revivification  in  place  was  the  out- 
come.   Two  methods  are  in  use  at  the  present  time,  namely,  ( 1 )  introduc- 


20  Gas  Purification  in  Medium  Size  Gas  Plants 

tion  of  a  small  percentage  of  air  (1  to  2  per  cent)  into  the  gas  ahead  of 
the  purifiers;  (2)  blowing  or  drawing  air  through  a  box  of  material  which 
has  been  shut  off  from  the  remainder  of  the  purifying  system.  Each  of 
these  methods  of  revivification  has  its  advantages  and  disadvantages,  and 
there  are  some  details  of  operation  in  both  cases  on  which  operators  dis- 
agree. 

SMALL  PERCENTAGE   OF  AIR  WITH   THE  GAS 

One  advantage  of  the  first  method  is  that  it  involves  no  danger  and 
requires  little  attention.  The  air  pump  is  connected  to  the  exhauster  and 
pumps  more  or  less  air  as  the  exhauster  runs  faster  or  slower.  One  obvious 
disadvantage  of  the  first  method  is  the  amount  of  inert  nitrogen  which  is 
introduced  into  the  gas,  if  an  attempt  is  made  to  introduce  enough  air  to 
secure  complete  revivification  in  place.  Although  less  than  0.5  per  cent  of 
air  would  theoretically  be  required  to  accomplish  the  revivification  of  an 
oxide  which  was  being  fouled  with  gas  containing,  say,  100  grains  of  H2S 
per  100  cubic  feet,  as  a  matter  of  fact,  even  2  per  cent  does  not  completely 
accomplish  revivification.  Excessive  nitrogen  of  course  dilutes  the  gas  and 
requires  additional  enrichment,  especially  if  the  gas  is  made  to  conform  to 
a  candle-power  standard.  With  the  heating-value  standard  which  is  for- 
tunately rapidly  replacing  the  candle-power  standard,  this  effect  is  not  so 
serious.  The  differences  in  operation  found  in  different  plants  with  this 
method  of  revivification  relate  chiefly  to  the  reversal  of  direction  of  gas 
flow  and  order  of  the  purifiers  with  respect  to  the  condition  of  the  con- 
tained materials.    These  will  be  discussed  in  the  next  section. 

AIR   BLOWN   THROUGH    OXIDE  IN   AN    OFF-BOX 

Revivification  by  forcing  air  through  a  box  of  oxide  after  shutting 
off  the  gas  has  been  employed  in  various  plants  for  a  number  of  years  and 
is  heartily  approved  by  many  operators,  and  as  heartily  condemned  by 
others.  Its  advantage  lies  in  the  fact  that  no  nitrogen  is  admitted  to  the 
gas.  On  the  other  hand,  careful  attention  has  to  be  given  to  it  while  in 
progress,  and  under  some  circumstances  it  may  be  dangerous.  O.  B. 
Evans1  of  Philadelphia  presented  a  paper  before  the  American  Gas  Associ- 
ation recently,  in  which  the  experiences  of  several  companies  in  the  use  of 
this  method  are  given.  From  these  experiences  he  concludes  that  revivifi- 
cation by  this  method  is  a  simple  operation  when  purifying  capacity  is 
ample  and  revivifications  are  frequent,  but  with  overloaded  purifiers 
extreme  care  must  be  used  to  prevent  firing  of  the  oxide.  He  believes  that 
the  best  method  is  to  recirculate  air  (which  soon  becomes  chiefly  nitrogen, 


10.  B.  Evans,  Revivification  in  place,  presented  at  a  meeting  of  the  Amer.  Gas  Assn.,  October, 
1919. 


Purifier  Operation  21 

since  the  oxygen  is  soon  removed  by  the  purifying  material)  continuously 
through  the  box  and  through  a  cooler  of  the  contact  type,  in  which  it  comes 
in  contact  with  water.  The  water  keeps  the  recirculated  mixture  saturated, 
and  the  water  vapor  helps  to  keep  down  the  temperature  in  the  oxide. 
Arrangement  of  valves  is  made  whereby  a  small  amount  of  fresh  air  can 
be  admitted  to  the  circulation  and  a  similar  amount  of  inert  gas  expelled 
from  it  as  desired.  The  blower  should  be  of  sufficient  capacity  to  reduce 
channeling  effect  and  to  circulate  the  mixture  faster  than  the  rate  at  which 
gas  is  passed  through  the  box  during  operation.  He  states  that  shallow 
boxes  aid  considerably  in  successful  revivification  in  place  on  account  of 
their  greater  radiating  capacity  per  bushel  of  oxide.  With  these  methods 
Mr.  Evans  believes  that  revivification  can  be  carried  out  without  danger, 
channeling  and  excessive  local  heating  being  largely  avoided.  The  suc- 
cessful experiences  of  many  operators  over  a  number  of  years  confirm  th;s. 

REVERSAL   OF   GAS    FLOW   AND    ROTATION   OF    BOXES 

Reversal  of  direction  of  gas  flow  through  the  purifiers  and  the  order 
of  the  various  purifiers  with  respect  to  the  condition  of  the  contained 
oxide,  are  matters  of  considerable  importance  in  connection  with  revivifica- 
tion in  place.  In  both  of  these  matters  the  operator  may  be  limited  by  the 
arrangement  of  his  equipment.  Not  all  plants  are  so  arranged  that  the 
direction  of  gas  flow  in  a  given  box  can  be  reversed;  and  while  a  certain 
amount  of  latitude  is  usually  allowed  as  to  the  order  of  purifying  boxes, 
one  finds  a  number  of  cases  where  the  connections  are  so  arranged  that  the 
possible  number  of  combinations  is  small.  Obviously,  any  group  of  puri- 
fiers must  be  so  connected  that  any  box  can  be  shut  off  for  refilling.  In 
most  of  the  older  installations,  when  one  box  is  off,  the  order  of  the  other 
boxes  is  predetermined,  only  one  arrangement  being  possible.  Reversal 
and  rotation,  as  applied  to  revivification  in  place,  depend  upon  the  principle 
pointed  out  several  years  ago  by  B.  E.  Chollar,  that  iron  sulphide  will  not 
revivify  to  the  oxide  in  the  presence  of  hydrogen  sulphide.  Let  us  assume, 
as  is  frequently  the  case  in  new  purifier  installations,  that  the  gas  enters  at 
the  middle  of  the  box  and  passes  downward  and  upward  through  the  two 
layers  of  oxide  and  comes  out  at  the  top  and  bottom  of  the  box.  The 
lower  layer  will  sulphide  downward  and  the  upper  layer  upward.  If  when 
the  sulphiding  has  extended  say  half-way  through  each  layer,  the  direction 
of  flow  be  reversed  so  that  the  gas  enters  at  the  top  and  bottom  of  the  box 
and  leaves  at  the  middle,  then  the  comparatively  fresh  oxide  in  the  top  of 
the  upper  layer  and  the  bottom  of  the  lower  layer  will  remove  the  H2S 
from  the  gas,  and  any  oxygen  present  will  go  on  and  revivify  the  foul 
upper  part  of  the  lower  layer  and  the  lower  part  of  the  upper  layer.     The 


22  Gas  Purification  in  Medium  Size  Gas  Plants 

frequency  of  reversals  will  depend  upon  the  degree  of  loading  of  the  puri- 
fiers. Where  the  purifiers  are  being  operated  at  normal  capacity,  reversal 
once  a  week  is  often  advised.  With  an  overload,  it  might  be  advisable  to 
to  reverse  oftener. 

Box  rotation  is  another  means  to  accomplish  the  same  end.  Where 
there  are  three  or  more  boxes  in  series,  it  would  seem  logical  to  have  the 
clean  box  first  to  remove  the  H,S  and  the  fouler  boxes  after,  to  be  revivified 
by  the  oxygen  which  had  been  admitted  to  the  gas.  The  following  are 
suggested  orders  of  rotation  of  a  4-box  set,  the  changes  being  made  when 
ITS  appears  at  the  outlet  of  the  third  box: 

1—2  —  3  —  4 

4—1—2  —  3 

3  —  4—1—2 

2  —  3  —  4—1 
In  this  way  the  most   revivified  batch   is  placed   first   and  the   next 
cleanest  batch  is  always  last  to  take  up  any  traces  of  H2S  which  may  get 
by  the  previous  batches  at  any  time. 

Where  there  are  only  two  boxes  in  series,  especially  those  of  the  non- 
reversing  type,  it  would  hardly  seem  advisable  to  have  the  clean  box  first, 
since  failure  of  the  second  box  to  revivify  for  any  reason  would  leave  no 
active  material  to  intercept  traces  of  hydrogen  sulphide. 

As  stated  earlier  in  this  section,  there  is  no  method  of  procedure  in  the 
matter  of  revivification  in  place,  which  is  accepted  as  best  practice  by  all 
operators.  The  only  way  by  which  any  operator  may  arrive  at  a  satisfac- 
tory conclusion  is  to  try  various  methods  and  arrangements  and  satisfy 
himself  which  method  is  most  applicable  to  his  particular  plant. 

Chemical  Control  and  Records 

In  order  to  secure  and  maintain  good  purifjang  efficiencies,  it  is  im- 
portant that  the  operator  know  at  all  times  the  status  of  the  material  in 
each  one  of.  his  purifiers.  Knowing  this,  he  will  not  only  be  able  to  judge 
whether  his  method  of  operation  is  satisfactory,  but  he  will  be  able  to 
detect  differences  in  purifying  material  which  might  otherwise  be  obscured 
by  other  conditions.  The  extent  of  the  system  of  tests  and  records  main- 
tained will  of  course  depend,  among  other  things,  upon  size  of  the  plant  and 
the  force  available.  Practically  every  plant  makes  the  simple  lead  acetate 
paper  test,  but  this  is  merely  qualitative.  It  tells  of  the  presence  or  absence 
of  hydrogen  sulphide,  but  gives  little  information  relative  to  the  amount 
present.  The  total-sulphur  test  carried  on  under  the  requirements  of  the 
Public  Utilities  Commission,  at  least  in  the  State  of  Illinois,  gives  informa- 


Purifier  Operation  23 

tion  only  with  respect  to  the  amount  of  sulphur  in  all  forms  present  in  the 
finshed  gas,  but  gives  no  information  in  regard  to  the  performance  of  the 
individual  purifiers. 

The  introduction  several  years  ago  of  the  Tutwiler  hydrogen  sulphide 
burette  marked  a  distinct  advance  in  the  matter  of  checking  up  purifier 
operation  because  it  gave  the  gas  operator  a  simple,  easy  method  of  study- 
ing his  purifier  performance  without  .the  aid  of  a  trained  chemist.  Un- 
fortunately, the  apparatus  has  to  be  made  of  glass,  and  is  so  shaped  and 
proportioned  that  it  is  easily  broken ;  and,  further,  unless  care  is  taken  to 
remove  the  plugs  of  the  glass  stop-cocks  and  insert  pieces  of  paper  around 
them  after  using,  they  are  very  liable  to  become  hopelessly  stuck  in  a  short 
time.  Nevertheless,  with  reasonable  care  the  apparatus  can  be  kept  in  good 
order,  and  the  information  obtained  with  it  is  very  useful.  One  advantage 
of  the  instrument  is  the  short  time  required  to  make  a  series  of  determina- 
tions. Probably  after  a  very  little  practice  any  operator,  even  with  no 
chemical  training  whatsoever,  could  make  a  test  of  the  gas  entering  the 
first  box  and  leaving  each  box  of  a  four-box  series,  in  fifteen  or  twenty 
minutes.  By  simple  subtraction  of  the  number  of  grains  of  hydrogen  sul- 
phide found  at  the  outlet  of  each  box  from  the  amount  present  at  the  inlet 
the  amount  of  sulphur  being  removed  by  each  box  would  be  immediately 
known.     The  results  of  a  test  on  a  certain  day  might  be  as  follows : 

H  ,S  Removed 

Grains  FLS  before  boxes ioo 

"         "     after  ist  box 25  75  by  1st  box 

"       2d       "    IO  l£    "    2d       " 

"    3d     "   5  5  "  3d     " 

"    4th    "   o  5  "  4th   " 

Now  5  grains  of  H2S  after  the  third  box  could  probably  be  detected 
by  lead  acetate  paper  as  ordinarily  used  and  would  perhaps  ordinarily  be 
taken  as  a  sign  to  change  the  order  or  empty  a  box,  but  if  this  same  distri- 
bution of  the  work  continued  for  several  days  and  the  last  box  handled  the 
remaining  few  grains  of  H.S  were  absorbed  in  the  last  box  all  right  it 
might  be  well  to  retain  this  order  for  a  time,  since  a  larger  absorption  in 
the  first  box  would  be  accumulating.  A  considerably  greater  sulphur 
absorption  might  be  realized  in  the  first  box  than  would  be  obtained  if  it 
were  emptied  or  reversed  as  soon  as  a  trace,  as  shown  by  lead  acetate  paper, 
was  visible  at  the  outlet  of  the  third  box.  Steere1  states  that  20  to  50 
grains  of  H2S  may  be  safely  passed  to  the  last  box  if  tests  are  made  regu- 
larly, and  the  boxes  are  properly  proportioned.  He  would  allow  20 
additional  grains  to  pass  to  the  last  regular  box,  where  a  catch  box  is  pro- 
vided. On  the  other  hand,  let  us  assume  that  the  following  results  were 
shown  by  the  tests: 


•Bull  37,  Steere  Engineering  Co.,    1919. 


24  Gas  Purification  in  Medium  Size  Gas  Plants 

Grains  H2S  before  boxes ioo 

ist   box 80  20  by  1st  box 

"  "  "      2d        "     20  60    "     2d       " 

"        "         "    3d      " 5  15   "   3d      " 

"    4th    "    o  5   "   4th    " 

It  would  at  once  be  evident  that  box  No.  1  was  not  doing  its  share  of 
the  work,  since  if  it  were  in  good  condition  it  could  usually  be  expected  to 
do  at  least  60  per  cent  of  the  total  absorption.     It  would  be  evident  that 
box  No.  2  was  bearing  the  chief  burden  and  it  would  be  high  time  to* 
empty  No.  1  or  reverse  so  that  it  would  clear  up. 

If  such  tests  as  the  above  were  made  faithfully  day  after  day  and 
carefully  recorded  in  a  book  (not  on  loose  scraps  of  paper)  it  is  evident  that 
a  running  record  could  be  maintained  from  which,  knowing  the  amount  of 
gas  metered  per  day,  the  actual  number  of  grains  of  H2S  absorbed  by  each 
box  from  change  to  change  would  be  known.  Knowing  the  number  of 
bushels  of  oxide  in  each  box,  the  volume  of  gas  passing  in  a  given  time,  and 
the  number  of  grains  of  H2S  absorbed  by  each  box  per  100  cubic  feet  of 
gas  passing  through,  it  would  be  very  simple  to  calculate  with  a  reasonable 
degree  of  accuracy  the  number  of  pounds  of  sulphur  absorbed  per  bushel. 
A  sample  record  and  computation  is  given  in  Appendix  B.  Such  results 
would  be  of  far  greater  value  in  determining  relative  merits  of  various 
operating  methods  or  of  various  oxides  than  would  the  usual  record  of  gas 
purified  per  bushel  between  changes,  because  unless  tests  are  made  there  is 
no  way  of  knowing  what  proportion  of  the  purification  should  be  credited 
to  each  box.  Usually  the  first  box  is  credited  with  all  of  the  purification 
on  the  theory  that  in  the  long  run  each  box  will  be  similarly  credited,  and 
will  average  up,  but  during  this  same  time  the  sulphur  content  of  the  gas 
may  change,  or  more  or  less  tar  may  be  carried  forward,  affecting  sulphur 
absorption  so  that  the  actual  performance  may  be  entirely  obscured  by 
other  conditions. 

As  a  final  check  on  operation,  analysis  of  the  oxide  for  suphur  and 
tar  after  each  removal  from  the  box  and  especially  before  discard  would  be 
very  helpful.  Often  a  batch  of  oxide  is  discarded  when  it  is  still  capable 
of  doing  much  useful  work,  and  perhaps  even  more  often  a  batch  is 
returned  to  the  box  at  considerable  expense  of  handling  when  it  might 
better  have  been  discarded.  Even  the  trained  eye  may  sometimes  be 
deceived  in  judging  oxide,  especially  if  it  contains  some  tar.  In  a  paper  by 
Fulweiler    and    Kunberger1    a    method    for    determining    mathematically 


'Some  of  the  Physical  Characteristics  of  Ferric  Oxide,  Proc,  Amer.   Gas  Institute,    1913,  Vol.  8, 
Pt.  1,  p.  476. 


Purifier  Operation  25 

whether  a  given  batch  of  oxide  is  worth  using  again  has  been  developed. 
The  formula  used  is  given  in  Appendix  C.  It  will  be  observed  that  no 
knowledge  of  higher  mathematics  is  required,  though  one  does  need  to 
know  what  the  batch  has  done  in  the  past  and  what  it  is  capable  of  doing 
as  judged  by  a  simple  laboratory  test.  If  the  operator  were  in  possession  of 
the  facts  derived  from  the  running  record  of  tests  above  described,  together 
with  certain  costs  which  he  should  know  for  intelligent  operation  of  his 
plant,  it  would  not  be  difficult  to  use  the  formula  provided  he  could  make 
or  have  made  an  analysis  of  his  oxide  and  a  laboratory  hydrogen  sulphide 
absorption  test. 

The  writers  of  this  paper  do  not  advise  every  gas  company  large  and 
small,  to  maintain  a  chemical  laboratory  and  a  trained  chemist.  The 
small  companies  probably  could  not  afford  these  refinements.  The  large 
companies  already  have  them  and  know  their  value.  We  will  not  attempt 
to  prescribe  the  minimum  size  of  plant  that  can  afford  a  laboratory.  How- 
ever, many  of  the  smaller  plants  in  this  and  other  states  are  links  in  a  chain 
of  plants.  It  would  seem  feasible  for  a  chain  of  several  plants  to  main- 
tain a  laboratory  and  a  chemist,  to  whom  samples  of  oxide,  ammonia,  and 
other  materials  could  be  sent  for  examination.  The  results  reported  by 
him  from  testing  oxides,  together  with  the  Tutwiler  test  made  by  the 
plant  superintendent  or  other  person,  and  adequate  record  of  purifier 
changes  and  batch  performance,  all  taken  together,  would  suffice  to  put 
purifying  operation  on  a  much  higher  plane  than  it  now  is  in  the  average 
plant.  It  is  believed  that  it  would  also  effect  a  real  economy  in  dollars 
and  cents  within  a  reasonable  time. 

Purification  costs  heretofore  have  usually  been  but  a  small  item  in  the 
total  cost  of  making  and  distributing  gas,  but  with  the  greatly  increased 
cost  of  labor,  it  is  becoming  especially  desirable  to  get  the  greatest  absorp- 
tion of  sulphur  with  the  least  amount  of  handling.  Revivification  in  place, 
with  its  obvious  advantages  where  properly  conducted,  should  become 
much  more  general.  If  difficulties  are  encountered,  careful  inquiries  should 
be  made  as  to  why  they  are  encountered.  Only  by  intelligent  study,  aided 
by  tests  and  good  records,  can  the  best  results  be  obtained. 

QUALITY  OF  OXIDE  FOR  GAS  PURIFICATION 

The  quality  of  oxide  used  for  purifying  gas  is  another  factor  affecting 
efficiency  and  economy  of  purification,  but  this  has  not  as  yet  been  worked 
out  so  that  it  can  be  expressed  mathematically  in  computing  performance 
of  a  given  equipment.  Some  research  work,  having  as  its  object  the  deter- 
mination of  the  effects  of  the  peculiar  properties  of  various  oxides  is  now 


26  Gas  Purification  in  Medium  Size  Gas  Plants 

in  progress,  and  it  is  hoped  to  throw  some  light  on  the  subject  in  a  later 
publication.  At  the  present  time  it  is  recognized  that  oxides  produced  by 
different  processes,  and  indeed  even  oxides  made  by  the  same  process,  show 
variations  in  performance,  but  just  what  are  the  causes  of  these  variations 
and  how  they  can  be  controlled  is  not  now  known. 

Types  of  Hybrated  Iron  Oxides 
Three  main  types  of  hydrated  oxide  of  iron  are  in  use  for  purification. 
These  include: 

( 1 )  Oxides  made  by  rusting  case  iron  borings  in  the  air  with  water 
only,  or  with  accelerating  agents  such  as  sulphate  of  iron,  salt,  etc. 

(2)  Natural  oxides,  which  include  certain  ores  having  the  proper 
chemical  and  physical  condition. 

(3)  Precipitated  oxides,*  made  by  the  chemical  precipitation  of 
hydrated  oxides  of  iron  from  the  salts  of  iron  produced  as  by-products 
in    certain    industries    or    from    the  iron-bearing    water    of    some    mines. 

Each  of  these  oxides  has  properties  peculiar  to  itself,  the  reasons  for 
which  are  not  yet  clear.  While  ferric  oxide  (Fe20;J>)  is  included  in  the 
composition  of  each  and  is  the  reacting  material,  the  presence  of  other 
materials  in  combination  with  it  and  the  physical  structure  of  the  material 
are  of  the  greatest  importance  in  determining  performance.  Ferric  oxide 
by  itself  without  water  of  hydration  is  almost  or  entirely  non-reactive  with 
hydrogen  sulphide. 

Formerly  the  iron  content  of  a  commercial  material  was  considered  as 
an  index  to  its  value  for  purifying  gas.  While  this  may  be  true  to  a  limited 
degree  when  applied  to  oxides  of  one  type,  it  does  not  hold  in  comparing 
oxides  of  different  types. 

Tests  of  Oxides 

Since  the  absorption  of  hydrogen  sulphide  is  the  main  thing  desired 
from  the  oxide,  it  follows  that  tests  which  will  indicate  the  absorbing  value 
of  a  particular  oxide  are  the  most  logical  ones  to  apply  in  valuing  a 
material.  Such  tests  have  been  devised  but  no  test  which  has  yet  been 
suggested  is  sufficiently  definite  in  its  provisions  and  indicative  of  the 
results  to  be  obtained  in  actual  practice  to  thoroughly  meet  the  require- 
ments of  a  standard  test.  This  is  evidenced  by  the  fact  that  the  Purifica- 
tion Committee  of  the  American  Gas  Association  is  now  endeavoring  to 
devise  such  a  standard  test,  which  will  meet  the  requirements  of  the  gas 
industry  generally. 

One  of  the  simplest  and  best-known  laboratory  tests  worked  out  thus 
far  is  that  of  A.  F.  Kunberger1    (see  Appendix  C).     In  the  Kunberger 


'Some  of  the  Physical  Characteristics  of  Ferric  Oxide:    Proc,  Amer.  Gas  Institute,   1013, 


Purifier  Operation  27 

method  a  small  weighed  sample  of  oxide  is  fouled  by  dry  hydrogen  sulphide 
for  one  hour,  the  water  liberated  by  the  reaction  being  retained  in  a  tube 
containing  granulated  fused  calcium  chloride,  which  is  weighed  with  the 
tube  containing  the  oxide  before  and  after  fouling.  The  gain  in  weight  of 
the  two  tubes  (or  one  tube  containing  both  oxide  and  calcium  chloride 
may  be  used)  is  equal  to  the  weight  of  H2S  absorbed. 

Such  a  test  is  very  useful  in  determining  the  relative  capacities  of 
various  materials  under  the  conditions  of  the  test.  The  test  is  also  said  by 
some  operators  to  check  practical  operations  quite  closely.  Other 
operators,  however,  place  less  reliance  in  it,  and  in  purchasing  an  oxide  of 
unknown  quality  are  not  satisfied  with  anything  less  than  a  semi-com- 
mercial test.  Such  a  test  usually  consists  in  fouling  two  oxides,  one  of 
known  practical  performance,  the  other  the  unknown  material,  with 
unpurified  gas,  in  a  pair  of  small  purifiers.  These  purifiers  may  contain 
from  a  few  quarts  to  a  few  bushels  of  the  materials  in  one  or  more  layers. 
Sometimes  two  sets  of  two  or  more  purifiers  each  are  used.  The  purifiers 
are  followed  by  gas  meters  to  measure  the  gas  passing  through  each  oxide. 
The  test  usually  consists  in  passing  gas  through  both  materials  at  the  same 
rate  and  noting  the  volume  of  gas  purified  by  each  until  the  time  when 
some  hydrogen  sulphide  passes  one  material  as  shown  by  a  test  with  lead 
acetate  paper  at  the  outlet.  The  rate  of  gas  flow  at  the  beginning  of  the 
test  is  generally  at  least  twice  that  usual  in  practice.  Sometimes  when  one 
material  begins  to  pass  hydrogen  sulphide  the  rate  of  flow  is  reduced  until 
absorption  is  complete  and  the  test  continued  at  the  new  rate  until  H2S 
again  passes  the  material.  The  test  may  be  continued  until  both  materials 
are  entirely  fouled  and  in  some  cases  the  materials  after  revivification  are 
tested  again.  Such  a  test  would  seem  to  possess  advantages  over  a  strictly 
laboratory  test,  since  the  same  kind  of  gas  is  used  that  has  to  be  purified  in 
the  works  purifiers  and  the  rate  of  fouling  is  nearer  to  that  obtaining  in 
practice.  On  the  other  hand,  in  small  test  installations  where  the  surface 
of  contact  between  the  oxide  and  the  box  is  usually  relatively  much 
greater  than  in  practice,  there  is  danger  of  the  gas  passing  up  the  sides  of 
the  box  to  a  considerable  extent  and  the  excessive  rates  of  purification  are 
likely  to  cause  channelling.  Another  disadvantage  of  tests  of  this  kind  is 
the  time  required  to  complete  them.  With  rates  of  fouling  approaching 
those  in  practice,  weeks  or  even  months  may  be  required  to  foul  the 
materials. 

In  all  tests  of  oxides,  the  great  difficult)-  is  to  interpret  the  results 
fairly  and  with  certainty.  At  present  those  gas  companies  who  make 
extensive  tests  on  oxides  interpret  the  tests  as  best  they  can  in  accordance 


28  Gas  Purification  in  Medium  Size  Gas  Plants 

with  their  own  particular  conditions  and  the  opinions  of  their  own 
engineers,  and  until  a  standard  test  with  carefully  specified  equipment  and 
testing  procedure  is  worked  out,  it  will  be  very  difficult  properly  to 
evaluate  commercial  purifying  materials. 

Activity  and  Capacity  of  Oxides 

The  existing  overloaded  condition  of  the  purifying  apparatus  in  many 
gas  plants  necessitates  more  attention  to  the  activity  or  speed  of  oxides  than 
has  hitherto  been  given  to  the  subject.  A  satisfactory  test  must  indicate 
the  relative  activity  of  the  materials  under  test.  At  the  same  time  capacity 
of  oxides  will  remain  an  important  consideration.  The  Steere  formula 
allows  about  6  minutes  time  of  contact  of  gas  with  oxide,  assuming  all  the 
space  occupied  by  the  oxide  to  be  free  space.  The  actual  free  space  in  a 
layer  of  oxide  will  of  course  be  considerably  less  than  the  total  volume, 
depending  upon  the  coarseness  of  the  material,  a  factor  which  is  continu- 
ally changing  as  the  material  is  used  and  becomes  more  and  more  clogged 
with  sulphur.  A  new  oxide  sponge  might  have  60  to  70  per  cent  free 
space,  and  therefore  the  actual  item  of  contact  would  be  considerably  less 
than  6  minutes  as  mentioned  above.  If,  as  is  sometimes  the  case,  the  time 
of  contact  in  a  heavily  loaded  purifying  system  is  reduced  to  2  minutes  or 
less,  it  is  evident  that  the  rapidity  of  the  material  may  have  a  very  im- 
portant bearing  on  its  value  for  such  a  plant.  It  is  quite  likely  that  various 
factors  could  be  worked  out  applying  to  various  types  of  purifying  oxides, 
which  introduced  into  formulas  for  purifying  capacities  would  materially 
alter  the  hourly  capacities  permissible  with  an  equipment  of  a  given  size 
and  arrangement.  This  subject  needs  further  study.  For  the  present  the 
differences  in  oxides  will  not  be  considered  in  studying  the  purifying  con- 
ditions in  Illinois  plants.  Computations  will  be  made,  assuming  that  all 
oxides  are  the  same.  In  drawing  conclusions  and  suggesting  remedies  for 
certain  cases,  the  possibility  of  using  more  rapid  oxides  must  be  borne  in 
mind. 

CONDITIONS  FOUND  IN  ILLINOIS  PLANTS 

Having  considered  the  conditions  affecting  purifying  efficiencies,  let 
us  see  what  the  actual  conditions  are  in  Illinois  plants,  so  far  as  can  be 
determined  from  the  information  collected. 

As  was  mentioned  earlier  in  this  bulletin,  sixteen  of  the  medium-size 
plants  of  the  State  were  visited.  Information  was  gathered  relative  to 
load  conditions,  size  and  kind  of  equipment,  operating  methods,  and  to  a 


Purifier  Operation  29 

certain  extent,  the  results  obtained.  This  latter  information  was  supple- 
mented by  the  results  of  analyses  of  spent  oxide  samples  collected  in  the 
various  plants. 

Upon  returning  to  headquarters  the  information  obtained  was  tabu- 
lated, purifier  capacities  were  computed,  oxide  samples  were  analyzed,  and 
the  results  tabulated.  Half  of  the  plants  visited  were  mixed-gas  plants. 
Their  problems  were  somewhat  different  from  those  of  straight  water-gas 
plants,  so  the  data  and  results  have  been  tabulated  separately.  In  accord- 
ance with  an  oral  agreement  under  which  results  were  obtained,  the  tabu- 
lations contain  no  plant  names.  The  writers  will  be  glad  to  inform  any 
operator  as  to  the  designation  of  his  own  plant,  in  order  that  explanations 
may  be  made  in  case  of  misunderstanding  or  difference  of  opinion  as  to 
results  obtained. 

Equipment  Conditions 

The  purifying  equipment  conditions  in  Illinois  plants  are  probably 
the  same  as  those  existing  in  similar  plants  elsewhere.  There  is  the  usual 
combination  of  old  and  new  equipment  which  results  from  piecemeal  con- 
struction. The  old  equipment  has  often  been  outgrown,  but  in  many  cases 
is  still  serviceable,  and  the  usual  policy  has  been  to  increase  capacity  by 
adding  another  purifier,  usually  of  the  outdoor  type,  to  the  older  indoor 
equipment,  retaining  the  latter  in  service.  Such  a  condition  exists  in  about 
half  of  the  plants  inspected. 

While  it  is  of  course  advantageous  to  get  as  much  economical  service 
as  possible  from  a  given  unit  of  equipment,  it  appears  in  some  cases  that 
the  old  equipment  is  a  drag  on  the  new.  In  making  additions  in  some 
cases,  the  minimum  cost  of  the  addition  has  been  looked  after,  rather  than 
operating  economy.  The  connections  are  so  arranged  as  to  permit  both 
the  old  and  new  equipment  to  be  used  at  the  same  time,  but  with  little 
regard  to  the  flexibility  of  the  system.  The  arrangement  is  often  so  fixed 
as  to  permit  only  one  order  of  the  boxes  in  series.  In  some  cases  there  is  a 
certain  latitude  for  arrangement  among  the  old  units,  but  usually  the  new 
equipment  either  precedes  or  follows  the  old  in  a  fixed  position  relative  to 
it.  Most  of  the  new  purifiers  are  equipped  for  deep  layers  of  oxide  and 
valved  for  reversible  flow,  whereas  most  of  the  old  boxes  have  shallow 
oxide  layers  and  straight  flow.  In  no  case  which  we  recall  is  there  a  com- 
bination of  old  and  new  equipment  in  use,  provided  with  arrangements 
for  perfect  flexibility  as  to  rotation  of  boxes  and  reversibility  of  flow.  In 
about  half  the  plants  are  the  boxes  uniform  as  to  size  and  type  and  com- 
pletely flexible  as  to  arrangement. 


30  Gas  Purification  in  Medium  Size  Gas  Plants 

The  computation  of  purifier  capacity  for  many  of  the  plants  is  a 
rather  complicated  matter.  The  Steere  formula  is  applicable  primarily 
to  boxes  arranged  for  reversible  flow,  though  the  formula  can  be  applied 
to  straight-flow  boxes  by  the  use  of  an  appropriate  factor.  In  some  plants 
where  there  is  a  combination  of  reversible  and  straight-flow  boxes,  it  is 
necessary  to  use  one's  judgment  in  the  selection  of  the  proper  factor,  and 
opinions  of  two  engineers  might  differ  as  to  the  proper  value  of  the  factor 
to  be  taken. 

Purifying  Equipment  of  Individual  Plants 

The  following  is  a  brief  description  of  the  purifying  equipment  of  the 
various  plants  inspected,  together  with  remarks  relative  to  special  condi- 
tions which  seem  noteworthy  and  pertinent. 

Plant  No.  1.  The  purifying  equipment  of  this  straight  water-gas 
plant  consists  of  three  cylindrical  steel  outdoor  boxes,  each  35  feet  in 
diameter  and  13  feet  high.  Each  box  contains  two  layers  of  oxide  4  feet 
deep.  The  arrangement  is  entirely  flexible,  permitting  the  boxes  to  be 
used  in  any  desired  order.  The  direction  of  gas  flow  in  each  box  is 
reversible,  the  gas  entering  between  the  layers  and  leaving  at  the  top  and 
bottom  or  vice  versa.  Southern  Illinois  coal  is  used  as  generator  fuel  in 
this  plant  and  the  gas  at  the  purifier  inlet  contained  180  grains  of  H2S 
per  100  cubic  feet  at  time  of  inspection.  The  computed  hourly  capacity 
of  the  purifiers  is  240,000  cubic  feet,  or  nearly  2.2  times  the  maximum 
hourly  make  reported.  The  installation  is  therefore  oversize  and  with 
good  oxide  would  permit  the  use  of  coal  of  considerably  higher  sulphide 
content,  if  other  conditions  made  it  desirable.  The  cost  of  purification  per 
thousand  cubic  feet  in  this  plant  should  be  very  low,  unless  the  capital 
charges  on  an  equipment  so  considerably  oversize  are  excessive.  No  figures 
relative  to  capital  charges  are  available.  Analyses  of  spent  oxide  from  thib 
plant  indicate  that  best  results  are  not  being  realized  from  this  equipment, 
the  sulphur  absorption  per  bushel  not  being  nearly  so  high  as  in  several 
other  water-gas  plants  that  are  much  more  heavily  loaded. 

Plant  No.  2.  The  purifying  equipment  in  this  plant  consists  of  two 
cylindrical  dry-seal  straight-flow  indoor  boxes,  18  feet  in  diameter  and  9 
feet  deep.  Each  box  contains  two  4-foot  3-inch  layers  of  oxide.  The 
generator  fuel  used  at  time  of  inspection  was  eastern  coke  and  the  H.,3 
content  of  the  unpurifled  gas  was  100  grains  per  100  cubic  feet.  The 
computed  hourly  capacity  of  the  boxes  was  39,800  cubic  feet  of  gas,  which 
could  theoretically  be  increased  to  about  52,000  cubic  feet  per  hour  by 
arranging  for  reversible  flow,  while  the  maximum  hourly  make  was  re- 
ported as  110,000  cubic  feet.     The  sulphur  absorption  per  bushel  in  this 


Purifier  Operation  31 

plant  was  low,  as  would  be  expected  in  a  plant  so  overloaded.  The  situa- 
tion was  complicated  by  insufficient  condensing  apparatus  and  by  a  relief 
holder  having  but  one  connection,  giving  but  slight  opportunity  for  any 
cooling  of  gas  in  the  holder.  At  the  time  of  inspection  the  gas  was  enter- 
ng  the  boxes  at  a  temperature  of  about  120°F.  and  was  carrying  much 
tar  tog.  Analyses  of  spent  oxide,  however,  indicate  that  this  is  not  a  year- 
around  condition.  The  purifying  condition  in  this  plant  were  the  most 
unfavorable  observed  in  the  State,  and  it  is  greatly  to  the  credit  of  the 
operators  that  they  succeeded  so  wTell  in  supplying  clean  gas  to  the  public. 
The  shortcomings  of  the  present  cleaning  and  purifying  s)rstems  are  realized 
'by  the  management,  and  extensive  improvements  are  projected. 

Plant  No.  3.  The  equipment  of  this  water-gas  plant  consists  of 
three  dry-seal,  oblong,  indoor  boxes  16  feet  by  12  feet  horizontal  section, 
and  12  feet  deep.  Each  box  contains  two  5-foot  layers  of  oxide.  These 
boxes  were  originally  designed  for  reversible  flow.  The  superintendent 
conceived  the  idea  that  the  capacity  would  be  increased  by  making  straight- 
flow  boxes  of  them.  The  computed  capacity  was  thereby  reduced  from 
55,200  to  40,800  cubic  feet  per  hour  with  gas  containing  190  grains  per 
100  cubic  feet.  Whether  decrease  in  efficiency  has  resulted  from  the 
change  would  be  extremely  difficult  to  determine,  since  recent  improve- 
ments in  the  tar-extracting  apparatus  have  probably  increased  efficiencies 
to  a  far  greater  degree  than  the  change  referred  to  decreased  them.  Since 
the  purifiers  are  now  overloaded  nearly  100  per  cent,  a  return  to  the  former 
arrangement  which  would  involve  very  little  expense,  is  suggested. 

Plant  No-.  4.  This  is  an  up-to-date  small  water-gas  plant.  The 
purifying  equipment  consists  of  two  cylindrical  reversible  How  outdoor 
boxes  14  feet  in  diameter  and  10  feet  high,  each  containing  two  layers  of 
oxide  4j/2  feet  deep.  The  computed  maximum  hourly  capacity  is  25,000 
cubic  feet  of  gas,  containing  175  grains  H..S  per  100  cubic  feet,  and  the 
reported  maximum  hourly  make  is  22,000.  Since  no  air  is  admitted  for 
revivification  the  reversibility  of  these  boxes  is  a  matter  of  less  consequence 
than  would  otherwise  be  the  case  and  the  actual  capacity  is  probably  less 
than  the  computed.  The  plant  is  seldom  operated  more  than  12  hours  per 
day.  In  addition  to  these  working  boxes  there  are  four  square  indoor 
water-seal,  straight-flow  boxes,  10  feet  in  diameter  and  4  feet  deep, 
designed  to  hold  one  3-foot  6-inch  layer  of  oxide  each.  These  boxes  are 
relics  from  a  former  coal-gas  plant.  They  are  connected  by  a  center  seal, 
so  that  only  three  of  them  can  be  in  service  at  one  time.  The  feasibility  of 
putting  these  boxes  into  service  so  as  to  permit  the  use  of  generator  fuel 
(bituminous)  of  higher  sulphur  content  has  been  considered.  Inasmuch 
as  the  coal  suggested  for  use  gives  about  400  grains  of  I I2S  in  the  gas,  and 


32  Gas  Purification  in  Medium  Size  Gas  Plants 

the  two  boxes  now  in  use  would  have  a  computed  maximum  hourly  capacity 
of  22,600  cubic  feet  of  gas  of  that  sulphur  content,  it  hardly  seems  that  the 
slightly  increased  hourly  capacity  to  be  obtained  from  the  small  old-type 
boxes  would  warrant  the  expense  of  putting  them  into  service  and  the 
inconveniences  attending  their  use. 

Plant  No.  5.  The  purifying  equipment  of  this  plant  was  originally 
designed  for  coal  gas.  Extensive  necessary  repairs  to  the  coal-gas  equip- 
ment, difficulty  of  securing  efficient  retort-house  labor,  and  the  economies 
realized  from  making  water  gas  from  bituminous  coal  have  resulted  in  at 
least  temporary  abandonment  of  the  coal-gas  equipment  except  in  so  far  as 
it  could  be  utilized  in  handling  the  water  gas.  The  purifiers  include  two 
oblong  dry-seal  indoor  boxes  15  by  10  by  8  feet  deep,  each  containing  two 
layers  of  oxide  Zy2  feet  deep,  and  one  square  dry-seal  box  20  by  20  by  5 
feet  deep,  containing  one  layer  of  oxide  4^  feet  deep.  The  boxes  are  all 
of  the  straight-flow  type.  The  large  shallow  box  can  be  used  only  as  a 
catch  box,  being  necessarily  last  in  the  series.  The  computed  capacity  of 
the  purifiers  is  32,700  cubic  feet  of  gas  per  hour,  and  the  overload  during 
hour  of  maximum  make  is  about  22  per  cent.  The  present  arrangement 
of  the  boxes  fulfills  present  needs  quite  well.  By  arranging  the  two  deep 
boxes  for  divided  and  reversible  flow,  the  capacity  could  be  increased  to 
about  36,500  cubic  feet  per  hour.  The  purifying  costs  reported  are  quite 
low. 

Plant  No.  6.  The  purifying  equipment  of  this  water-gas  plant  con- 
sists of  two  rectangular  indoor  boxes  16  by  12  by  IIV2  ^eet>  eacn  contain- 
ing two  5-foot  layers  of  oxide,  in  parallel  with  two  boxes  20  by  20  by  5 
feet  containing  one  4-foot  layer  of  oxide  each.  The  deeper  boxes  are 
arranged  for  reversible  flow,  but  the  shallow  boxes  have  straight  flow  only. 
The  computed  capacity  of  the  system  as  now  arranged  is  about  60,000 
cubic  feet  per  hour.  The  overload  at  the  time  of  maximum  make  is  about 
67  per  cent.  It  is  understood  that  a  rearrangement  of  the  boxes  whereby 
all  the  boxes  would  be  in  series,  has  been  considered.  If  such  a  change 
were  practicable  from  other  considerations,  the  capacity  would  be  increased 
slightly  but  probably  not  sufficiently  to  pay.  On  the  other  hand,  if  plenty 
of  overhead  room  is  available,  and  it  were  practicable  to  double  the  depth 
of  the  two  shallow  boxes,  making  two  layers  in  each  box,  and  to  arrange 
the  whole  system  for  reversible  flow  throughout,  the  capacity  would  be 
increased  to  over  100,000  per  hour  or  to  about  the  present  maximum 
hourly  production  of  the  plant. 

Plant  No.  7.  This  large  suburban  water-gas  plant  is  equipped  with 
four  rectangular  water-sealed  indoor  boxes  each  containing  two  5-foot 
layers  of  oxide.    The  gas  flow  in  each  box  is  reversible,  gas  entering  at  the 


Purifier  Operation  33 

middle  and  leaving  at  the  top  and  the  bottom  of  the  box  or  vice  versa. 
Rotation  of  the  boxes  can  be  accomplished  in  only  one  direction,  as:  1  — 
2  —  3  —  4,  2  —  3  —  4—1,  etc.,  but  not  4  —  3  —  2—  1,  etc. 
The  computed  hourly  capacity  of  the  system  is  about  206,000  cubic  feet. 
It  is  about  40  per  cent  overloaded  at  time  of  maximum  production.  The 
purifying  costs  in  this  plant  are  the  lowest  reported  by  any  plant  inspected. 
The  spent  oxide  from  this  plant  showed  a  sulphur  content  rather  above 
the  average.  The  tar  content  was  higher  than  the  average  found  in  all 
the  plants  and  indicated  that  an  excessive  amount  of  tar  had  been  allowed 
to  enter  the  boxes  at  some  time  during  the  life  of  the  oxide.  At  the  time 
of  inspection,  the  gas  entering  the  boxes  was  reasonably  clean.  The  indi- 
cations are  that  purifying  results  could  be  improved  somewhat  if  the  load 
curve,  shown  in  Figure  2,  could  be  smoothed  out,  making  the  production 
through  the  boxes  more  uniform. 

Plant  No.  8.  This  suburban  water-gas  plant  has  two  sets  of  purify- 
ing boxes  in  parallel.  One  set  consists  of  four  rectangular  indoor  boxes 
connected  by  a  center  seal  which  permits  the  use  of  only  three  boxes  at  a 
time.  Each  box  is  24  by  24  by  7  feet  deep  and  contains  two  layers  of 
oxide  3  feet  deep.  The  other  set  of  boxes  consists  of  four  boxes,  each  16 
by  16  by  4!/2  feet  deep.  Each  box  contains  a  single  layer  of  oxide  21/) 
feet  deep.  Three  of  the  boxes  are  interchangeable,  viz.,  any  one  of  them 
can  be  made  first  box,  but  the  position  of  one  box  fixes  the  order  of  the  set. 
The  fourth  box  of  the  set  is  a  catch  box  and  is  always  at  the  end  of  the 
series.  All  the  boxes  in  both  sets  are  arranged  for  straight  upward  flow. 
The  computed  maximum  capacity  of  the  two  sets  of  boxes,  as  now 
arranged,  is  about  100,000  cubic  feet  of  gas  per  hour.  The  capacities  of 
the  two  sets  are  so  different  (about  4  to  1 )  that  any  rearrangement  by 
placing  in  series  would  probably  not  be  feasible.  The  set  of  larger  boxes 
has  a  present  capacity  of  about  80,000  cubic  feet  of  gas  per  hour.  If  a 
new  valving  system  could  be  installed,  putting  the  fourth  box  into  use  and 
permitting  reversal  of  flow,  the  capacity  of  the  set  would  be  approximately 
doubled,  giving  the  entire  system  a  total  capacity  of  180,000  cubic  feet, 
more  or  less,  per  hour,  which  is  about  equal  to  the  present  maximum  flow 
through  the  purifiers. 

Plant  No.  9.  This  plant  produces  about  60  per  cent  water-gas  and 
40  per  cent  coal-gas.  Each  kind  of  gas  has  an  independent  condensing 
system  and  the  gases  are  mixed  at  the  inlet  of  the  purifiers.  The  purify- 
ing equipment  consists  of  six  boxes.  Of  these,  four  indoor  boxes  are 
arranged  in  two  pairs,  the  members  of  each  pair  being  connected  in  parallel 
and  each  pair  acting  as  one  divided-flow  but  non-reversible  box.  The 
paired  boxes  are  16  feet  square  in  horizontal  section  and  contain  one  layer 


34  Gas  Purification  in  Medium  Size  Gas  Plants 

of  oxide  each,  three  feet  deep.  The  remaining  boxes  include  an  outdoor 
cylindrical  steel  box  25  feet  in  diameter  and  10  feet  high,  containing  two 
4-foot  layers  of  oxide,  and  an  indoor  catch  box  16  by  16  by  4  feet  deep,  con- 
taining one  4-foot  layer  of  oxide.  The  paired  units  and  the  cylindrical  box 
can  be  arranged  in  various  orders  asl  —  2  —  3,  3  —  1  —  2,  and 
2  —  3  —  1.  The  direction  of  flow  except  in  the  cylindrical  box  is  non- 
reversible. The  history  of  the  development  of  this  installation  is  not 
known.  It  seems  likely  that  the  original  installation  consisted  of  a  four- 
box  set  arranged  with  a  center  seal  as  is  common  in  old  installations.  In 
adding  to  the  original  equipment,  an  arrangement  was  made  which  is  more 
flexible  than  is  found  in  many  plants.  The  computed  hourly  capacity  is 
about  70,000  cubic  feet  per  hour,  which  is  ample  for  present  needs. 

Plant  No.  10.  This  mixed-gas  plant  puts  out  about  90  per  cent  coal 
gas  on  the  average.  The  peak  load  on  the  purifiers  is  rather  sharp,  the 
maximum  hourly  production  being  about  9.5  per  cent  of  the  maximum 
daily  production.  The  purifiers  include  two  cylindrical  outdoor  boxes  15 
feet  in  diameter  and  12  feet  high  arranged  for  reversible  flow.  Each  box 
contains  two  5-foot  layers  of  oxide.  The  computed  maximum  capacity  is 
about  30,000  cubic  feet  per  hour  and  since  the  maximum  hourly  produc- 
tion through  the  boxes  is  said  to  be  75,000  cubic  feet,  there  is  a  large  load 
factor.  The  storage  capacity  in  this  plant  is  about  65  per  cent  of  the  max- 
imum day.  Water-gas  and  coal-gas  are  cleaned  separately  and  mixed  at 
the  inlet  of  the  boxes.  The  coal-gas  production  will  probably  average 
around  540,000  cubic  feet  per  day,  with  an  hourly  production  of  about 
20,000  cubic  feet.  The  maximum  daily  gas  output  from  the  plant  is 
reported  to  be  about  800,000  cubic  feet,  which  indicates  that  the  maximum 
water-gas  production  may  be  about  32.5  per  cent.  The  overload  on  the 
purifiers  then  is  evidently  due  to  the  pumping  of  water-gas  through  the 
purifiers  at  an  exceptionally  high  rate  during  peak  load.  It  would  seem, 
however,  that  if  the  maximum  hourly  output  of  the  plant  for  a  peak  lasting 
say  four  hours  at  a  time  averaged  no  greater  than  the  reported  maximum 
make  through  the  boxes,  namely,  75,000  cubic  feet  per  hour,  and  if  the 
holders  were  full  at  the  beginning  of  the  peak,  there  would  be  no  necessity 
of  pumping  gas  through  the  purifiers  so  fast,  even  were  it  ncessary  to  retain 
the  city  holder  two-thirds  full  at  all  times.  Even  were  the  load  curve 
smoothed  out  as  much  as  possible,  with  the  existing  storage  capacity  it  is 
likely  that  there  would  be  a  considerable  overload  during  maximum  hours. 
The  logical  extension  of  the  purifying  system  would  seem  to  be  the  instal- 
lation of  another  box  similar  to  those  now  in  place.  Another  such  box 
would  bring  the  hourly  capacity  up  to  about  50,000  cubic  feet  per  hour. 


Purifier  Operation  35 

Plant  No.  11.  This  small  mixed-gas  plant  has  an  average  output  of 
about  200,000  cubic  feet  of  gas  per  day,  of  which  about  one-fourth  is 
water-gas.  The  maximum  hourly  purification  is  about  12,000  cubic  feet. 
The  purifying  equipment  consists  of  two  cylindrical  outdoor  purifiers,  15 
feet  in  diameter  and  12  feet  high,  containing  two  5-foot  layers  of  oxide 
each.  The  flow  is  divided  and  reversible.  The  computed  capacity  of  these 
purifiers  is  about  30,000  cubic  feet  per  hour ;  therefore  the  system  is  much 
underloaded.  It  would  be  interesting  to  know  the  capital  charges  on  an 
oversize  system  of  this  kind,  but  no  figures  are  available.  The  size  of  the 
purifiers  is  ample  to  care  for  the  growth  in  output  for  some  years  to  come. 

Plant  No.  12.  The  purifying  equipment  of  this  plant  which  produces 
only  10  to  15  per  cent  water-gas,  consists  of  four  water-sealed  indoor  boxes 
16  by  16  by  7.5  feet  deep,  each  containing  two  layers  of  oxide  2.75  feet 
deep.  Three  of  the  boxes  are  arranged  for  rotation,  the  possible  arrange- 
ments being  1  —  2  —  3,  2  —  3  —  1,  3  —  1  —  2,  but  not  the  reverse. 
The  fourth  box  acts  as  a  catch  box  and  is  always  in  last  position.  The 
computed  capacity  of  this  installation  is  about  44,800  cubic  feet.  If  the 
boxes  were  valved  for  divided  reversible  flow,  and  2  per  cent  of  air  used 
for  revivification  in  place,  the  computed  capacity  would  become  about 
61,000  cubic  feet  per  hour.  At  present  the  installation  is  about  25  per  cent 
overloaded  at  time  of  maximum  hourly  production,  but  since  the  load  is 
being  handled  well  and  the  cost  of  purification  is  low,  there  is  little  reason 
for  making  a  change.  The  use  of  higher  sulphur  coals,  if  desirable  or 
necessary  for  other  reasons,  might  make  the  suggested  change  advisable. 

Plant  No.  13.  The  original  purifying  equipment  of  this  plant,  which 
makes  about  40  per  cent  water-gas,  consisted  of  two  rectangular,  water- 
sealed,  indoor  boxes,  each  16  by  24  by  5  feet  deep  and  containing  one 
layer  of  oxide,  4  feet  4  inches  thick.  A  cylindrical  outdoor  box,  15  feet  in 
diameter  and  12  feet  high,  containing  two  5-foot  layers  of  oxide  was  subse- 
quently installed.  The  new  box  is  of  the  divided-flow  reversible  type.  On 
account  of  tar  trouble,  the  first  rectangular  purifier  was  emptied  and 
refilled  with  shavings  to  act  as  a  shavings  scrubber.  The  remaining  rect- 
angular box  and  the  new  outdoor  box  have  a  combined  capacity  of  about 
26,000  cubic  feet  of  gas  per  hour.  As  is  common  in  piece-meal  installa- 
tions of  this  kind,  there  is  little  opportunity  for  rotation  of  boxes;  indeed 
in  this  plant  there  is  only  one  arrangement  possible.  The  rectangular  box 
is  always  first  in  series,  and  the  only  change  possible  is  reversal  of  flow  in 
the  second  box.  The  overload  during  maximum  hour  is  only  about  15 
per  cent  and  there  seems  little  reason  for  making  a  change  on  that  account. 
Greater  flexibility  with  respect  to  rotation  of  boxes  would  be  desirable  and 
would  probably  permit  a  more  nearly  complete  fouling  of  the  oxide  and  a 


36  Gas  Purification  in  Medium  Size  Gas  Plants 

reduction  in  purification  labor.  If  a  regular  shavings  scrubber  were 
installed  and  the  box  now  used  for  tar  extraction  were  put  into  service 
again  as  a  purifier,  the  capacity  of  the  installation  would  be  increased  to 
about  35,000  cubic  feet  of  gas  per  hour. 

Plant  No.  14.  This  plant  makes  about  95  per  cent  coal-gas  and  the 
purifying  results  obtained  are  considerably  above  the  average.  Seven  boxes 
are  in  use,  of  which  three  boxes  handle  coal-gas  exclusively,  while  the 
remaining  four  handle  a  smaller  part  of  the  coal-gas  and  all  the  water-gas. 
The  purifiers  are  arranged  in  two  parallel  groups.  One  group  consists  of 
two  24  by  24  by  5  foot  boxes,  each  box  containing  one  layer  of  oxide  4  feet 
6  inches  in  depth,  and  one  24  by  24  by  12  feet  box  containing  two  oxide 
layers,  each  4  feet  6  inches  in  depth.  These  boxes  are  straight  flow,  but 
the  sequence  can  be  changed.  The  other  group  of  boxes  consists  of  four 
rectangular  boxes,  each  20  feet  in  diameter  and  containing  one  2  foot  6  inch 
layer  of  oxide.  These  boxes  are  connected  by  a  center  seal  and  one  box  is 
always  off.  The  computed  hourly  capacity  of  the  first  group  is  about 
72,000  cubic  feet-  of  gas  per  hour,  and  that  of  the  second  group  about 
23,000  cubic  feet  per  hour.  Since  the  maximum  hourly  production  is 
reported  to  be  70,000  cubic  feet,  the  system  is  not  overloaded,  at  least  with 
gas  of  the  present  sulphur  content.  In  case  an  increased  capacity  were 
necessary,  it  might  be  desirable  to  build  up  the  two  shallow  24-foot  boxes 
to  conform  with  the  existing  deep  box  of  the  same  area.  Such  an  enlarge- 
ment with  valves  so  arranged  as  to  give  complete  reversibility  would 
increase  the  hourly  capacity  of  this  group  from  72,000  cubic  feet  to  157,000 
cubic  feet  per  hour,  over  150  per  cent  of  the  present  total  capacity.  Sim- 
ilarly, the  four  shallow  boxes  of  the  other  group,  if  built  up  to  double  depth 
and  arranged  for  complete  reversibility,  would  have  a  capacity  of  about 
100,000  cubic  feet  per  hour. 

Plant  No.  15.  This  small  mixed-gas  plant  produces  about  40  per 
cent  water-gas.  The  purifying  equipment  consists  of  four  old-type,  indoor, 
water-sealed  boxes,  connected  by  a  center  seal  which  permits  the  use  of 
only  three  boxes  at  one  time.  The  boxes  are  10  by  8  by  4  feet  and  each  con- 
tains one  3-foot  6-inch  layer  of  oxide.  The  calculated  capacity  of  the 
installation  as  now  arranged  is  only  about  6,000  cubic  feet  per  hour,  while 
the  maximum  load  is  reported  to  be  15,000  cubic  feet,  and  the  average  load 
7,000  cubic  feet  per  hour.  It  is  evident,  therefore,  that  the  installation  is 
heavily  overloaded.  If  the  fourth  box  could  be  put  into  service  except  for 
the  short  interval  required  to  change  a  box,  the  capacity  would  be  increased 
to  about  7,500  cubic  feet  per  hour,  which  would  still  leave  100  per  cent 
overload.  A  new  purifying  system,  or  the  addition  of  one  up-to-date  box 
with  proper  valving,  would  perhaps  be  the  best  solution.     The  present 


Purifier  Operation  37 

single  boxes,  if  built  up  to  double  height  and  arranged  for  complete  revers- 
ibility of  flow,  would  have  a  computed  hourly  capacity  of  about  22,600 
cubic  feet.  Such  conditions  as  head  room  available,  strength  of  supports 
of  the  present  system,  condition  of  present  boxes,  and  cost  of  the  improve- 
ments would  of  course  determine  the  best  way  to  increase  capacity.  The 
fact  that  the  purifying  labor  cost  per  1,000  cubic  feet  in  this  plant  is  about 
double  that  of  any  other  plant  inspected  (other  small  plants  included) 
indicates  that  there  is  sufficient  economy  to  be  realized  in  labor  charges  to 
offset  a  considerable  capital  charge. 

Plant  No.  16.  This  plant  makes  about  93  per  cent  coal-gas.  The 
purifying  equipment  is  rather  unique  among  the  installations  inspected. 
The  system  includes  two  outdoor  concrete  boxes,  each  40  by  26  by  14  feet 
4%  inches  deep,  each  containing  one  53-inch  layer  and  one  62-inch  layer, 
two  indoor  boxes  each  30  by  20  by  5  feet  deep,  containing  one  4^-foot 
layer,  and  two  indoor  boxes  20  by  20  by  4%  feet  deep  containing  one  3-foot 
9-inch  layer  each.  The  greater  part  of  the  purification  is  done  in  the  two 
concrete  boxes.  These  are  placed  first  in  the  series  and  the  order  cannot 
be  changed,  nor  is  there  any  rotation  of  the  two  boxes  though  they  are 
valved  for  divided  reversible  flow.  Coal-gas  only  is  purified  in  the  con- 
crete boxes.  The  old  type  indoor  boxes  follow  the  concrete  boxes  and  the 
water-gas  enters  the  inlet  of  the  former.  The  old  purifiers  are  all  single- 
layer  straight-flow  boxes  and  are  all  arranged  in  series.  The  computed 
capacity  of  the  whole  system  is  about  190,000  cubic  feet  of  gas  per  hour 
while  the  maximum  hourly  purification  is  reported  to  be  only  40,000  cubic 
feet.  It  is  evident,  therefore,  that  the  capacity  is  ample  for  some  time  to 
come.  This  plant  was  the  only  one  inspected  in  which  the  oxide  was 
revivified  in  the  box  by  an  air  blast.  Only  the  oxide  in  the  large  concrete 
boxes  could  be  revivified  in  this  manner.  The  way  in  which  revivification 
is  conducted  in  this  plant  will  be  discussed  later. 

Summary  of  Capacities  and  Load  Conditions 

Tables  1  and  2  give  the  summarized  data  relative  to  load  conditions, 
storage  capacity,  purifier  capacity,  etc.,  of  the  plants  visited. 


38 


Gas  Purification  in  Medium  Size  Gas  Plants 


Table  i. — Purifier  load  conditions  in  medium-size  water-gas  plants  of  Illii 


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2210 

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6285 

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3880 

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161 

280 

C  -  5000 
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17125 

2865 

206 

136.0 

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125 

10900 

2660 

104 

168.8 

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141 

5686 

145.8 

72.4 

1  In  holder  capacity  column,  the  abbreviations 
Holder,  respectively. 


'C"  and  "R"  stand  for  City  Holder  and  Relief 


Purifier  Operation 

Table  i. — Purifier  load  conditions  in  medium-si^e  mixed-gas  plants  of  Illinois. 


39 


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5960 

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83 

350 

675 

3370 

6 

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121.0 

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1400 

1800 

58 

65 

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R  -     390 

77 

470 

22805 

12700 

191 

34.0 

32.9 

Aa 

82 

252 

6680 

120.6 

61.0 

iln  holder  capacity  column,  the  abbreviations  "C"  and  "R"  stand  for  City  Holder  and  Relief 
Holder,  respectively. 

In  studying  Tables  1  and  2  several  outstanding  facts  will  be  noted.  Of 
the  eight  water-gas  plants,  six  are  loaded  at  maximum  hour  beyond  the 
rated  capacities  of  their  purifiers,  while  on  the  other  hand,  only  two  of  the 
eight  are  overloaded  during  hours  of  average  production.  It  is  evident 
then  that  the  lack  of  uniformity  of  load  is  the  chief  cause  of  overload.  It 
will  be  noted  that  the  maximum  hourly  make  is  often  more  than  double 
the  average  hourly  make.  (By  "make"  here,  is  meant  flow  through  puri- 
fiers, not  machine  make.)  By  referring  to  the  column  marked  "Calculated 
max.  hrly.  capacity  of  purifiers"  and  "Bushels  of  oxide  per  million  cu.  ft., 
max.  day",  we  are  able  to  form  some  opinion  as  to  the  reason  for  the  over- 
loaded or  underloaded  condition  in  each  case.  It  is  interesting  to  note  that 
the  average  number  of  bushels  of  oxide  per  million  cubic  feet  of  gas  purified 
on  the  maximum  day,  namely  5,686,  agrees  quite  closely  with  the  figure 
5,365,  which  is  the  average  of  twenty-three  water-gas  plants  whose  results 
are  reported  by  O.  B.  Evans  in  his  paper,  "Revivification  in  place",  already 
mentioned.  This  indicates  that  Illinois  plant  conditions  conform  quite 
closely  to  conditions  in  plants  all  over  the  country,  as  is  to  be  expected. 


40  Gas  Purification  in  Medium  Size  Gas  Plants 

While,  as  shown  in  the  tables,  the  holder  capacity  bears  no  direct 
relation  to  purifying  capacity,  it  will  be  noted  that  in  plants  where  the 
volume  of  oxide  is  small,  large  holder  capacity  assists  in  keeping  down  the 
overload.  Plants  1  and  2  are  interesting  as  the  extreme  examples  of  pre- 
vailing conditions.  These  plants  have  approximately  the  same  output. 
Plant  1,  however,  has  a  holder  capacity  greater  than  its  maximum  day  and 
an  oxide  capacity  per  million  cubic  feet  of  gas  nearly  twice  the  average. 
Plant  2,  on  the  other  hand,  has  a  holder  capacity  only  30.6  per  cent  of  its 
maximum  day  and  an  oxide  capacity  less  than  half  the  average.  It  is  sur- 
prising to  find  that  despite  these  great  differences,  the  amount  of  sulphur 
absorbed  per  bushel  by  both  of  these  oxides  is  almost  the  same  with  a  very 
slight  advantage  in  favor  of  Plant  1.  The  probable  reasons  for  this  sim- 
ilarity of  results  under  such  different  conditions  will  be  discussed  later  in 
connection  with  Table  3.  Plant  No.  8  (see  Table  1)  presents  an 
apparently  anomalous  condition.  Here  the  holder  capacity  is  only  14.6  per 
cent  of  the  maximum  day  and  there  is  less  than  half  the  average  amount 
of  oxide  per  million  cubic  feet  on  maximum  day  found  in  all  the  plants, 
yet  the  overload  is  not  nearly  so  great  as  in  some  other  plants  which  are 
apparently  more  favored.  The  reason,  however,  seems  to  lie  in  the  connec- 
tion of  this  plant  with  a  larger  system  whose  holder  capacity  is  to  a  certain 
extent  available  for  use  by  this  plant.  In  this  case  it  is  practically  impos- 
sible to  say  just  what  average  holder  capacity  is  available  for  Plant  No.  8. 
though  its  own  individual  holder  capacity  is  as  given.  Probably  at  least 
four  times  the  capacity  stated  is  actually  available  when  needed.  Formerly 
many  gas  engineers  considered  that  a  well-designed  plant  should  have  a 
holder  capacity  at  least  equal  to  its  maximum  day,  but  the  rapid  growth  of 
output  in  most  of  our  plants  has  resulted  in  much  smaller  storage  capacity 
ratios,  as  shown  in  Tables  1  and  2.  When  one  considers  that  in  spite  of 
the  smaller  margin  of  safety  which  a  small  holder  capacity  gives,  practically 
all  of  the  larger  plants  operate  year  after  year  and  give  the  public  uninter- 
rupted service,  one  is  likely  to  conclude  that  there  is  no  well-defined  lower 
limit  beyond  which  the  gas  operator  dare  not  go.  While  this  is  true  to  a 
certain  extent  and  distribution  conditions  are  likely  to  be  the  determining 
factor  in  dictating  an  increase  in  storage  capacity,  it  is  evident  that  this 
condition  is  reflected  back  upon  plant  operation  and  affects  purification,  tar 
removal,  and  indeed  any  phase  of  the  gas  manufacturing  process  in  which 
rate  of  gas  flow  is  a  factor. 

In  Table  2,  the  purifier  load  conditions  in  eight  mixed-gas  plants  are 
presented.  It  is  to  be  noted  that  the  percentage  of  water-gas  varies  from 
5  to  60  per  cent  of  the  total,  averaging  25  per  cent  in  all  plants.  It  will 
be  observed  that  the  average  holder  capacity  is  larger  in  proportion  to  the 


Purifier  Operation  41 

maximum  daily  output  than  in  the  straight  water-gas  plants,  as  is  also,  in 
general,  the  volume  of  oxide  per  million  cubic  feet  output  on  maximum  day. 
Only  one  of  the  mixed-gas  plants  had  an  overload  on  its  purifiers,  dur- 
ing hours  of  average  production.  Since  the  coal-gas  production  in  a  mixed 
gas  plant  is  usually  fairly  uniform  in  rate,  the  water-gas  production  taking 
the  peak  load,  it  is  evident  that  in  most  cases  where  the  purifiers  are  over- 
loaded,the  overload  is  caused  by  the  production  and  purification  of  a  large 
volume  of  water-gas  in  a  short  time.  The  purifiers  are  usually- designed  to 
handle  the  coal-gas  production,  but  the  water-gas  production  has  often  not 
been  provided  for.  It  will  be  noted  that  the  hydrogen  sulphide  content  of 
the  gas,  while  about  double  that  found  in  the  eight  water-gas  plants,  is 
considerably  lower  than  in  usual  coal-gas  practice,  even  with  very  low- 
sulphur  coals.  Had  the  sulphur  content  been  higher,  then  the  average 
purifying  capacity  would  have  shown  an  overload  according  to  the  rela- 
tion shown  in  Figure  1. 

EFFECT  ON  PURIFIER  CAPACITIES  OF  THE  USE  OF 
ILLINOIS  COAL 

In  connection  with  the  influence  of  sulphur  content  of  the  gas  on  puri- 
fying capacity,  it  is  interesting  to  note  the  effect  of  a  change  from  the  low- 
sulphur  fuels  used  in  a  majority  of  the  plants  at  the  present  time  to  fuels  of 
higher  sulphur  content.  The  water-gas  plants  listed  in  Table  1,  with  the 
exception  of  Plants  2  and  8,  were  using  Illinois  or  Indiana  coals  as  genera- 
tor fuel  at  the  time  of  inspection.  These  two  plants  were  using  eastern 
cokes.  The  coals  were  in  all  cases  selected  low-sulphur  coals.  It  will  be 
noted  that  in  some  of  the  plants  using  coal,  the  H2S  content  of  the  gas  is 
no  higher  than  in  those  using  coke.  In  general,  however,  the  increase  in 
sulphur  content  with  Illinois  or  Indiana  low-sulphur  coals  as  generator 
fuel  would  be  from  50  to  100  per  cent,  viz.,  an  H2S  content  in  the  gas  of 
150  grains  to  200  grains  per  100  cubic  feet  might  usually  be  expected.  By 
reference  to  Figure  1  it  will  be  noted  that  an  increase  of  50  to  100  per 
cent  in  the  H2S  content  of  the  gas  does  not  have  nearly  the  effect  on  the 
rated  purifier  capacity,  when  the  H2S  content  is  small,  that  a  similar  per- 
centage increase  would  have  if  the  ITS  content  were  large.  Hence  a 
change  from  a  low-sulphur  eastern  coke  as  water-gas  generator  fuel  to  a 
low-sulphur  central  district  coal  does  not  have  as  great  an  effect  as  the 
change  from  a  low-sulphur  eastern  gas  coal  to  a  low-sulphur  Illinois  coal 
would  have  in  coal-gas  manufacture,  it  being  assumed,  as  is  usually  the 
case,  that  the  best  eastern  gas  coals  will  average  lower  in  sulphur  content 
than  the  best  Illinois  coals.  It  is  interesting  to  take  numerical  examples 
to  study  the  effect  of  such  changes. 


42  Gas  Purification  in  Medium  Size  Gas  P 


LANTS 


Assume  that  an  eastern  coke  as  generator  fuel  gives  gas  containing 
100  grains  of  H2S  per  100  cubic  feet,  and  that  a  central  district  coal,  using 
the  same  oil  for  enrichment  gives  gas  containing  200  grains  per  100  cubic 
feet.  A  plant  which  was  designed  to  purify  100,000  cubic  feet  of  gas  of 
the  lower  sulphur  content  per  hour  would  have  its  capacity  reduced  to 
96,000  cubic  feet— a  decrease  of  only  4  per  cent.  On  the  other  hand,  if  a 
low-sulphur  eastern  gas  coal  gave  300  grains  of  H2S  at  the  inlet  to  the 
purifier  and  an  Illinois  coal  gave,  as  reported  in  some  cases,  600  grains  of 
H2S,  then  a  plant  equipped  to  handle  100,000  cubic  feet  of  the  lower- 
sulphur  gas  per  hour  would  have  its  capacity  reduced  by  the  change  to 
about  83,000  cubic  feet, — a  decrease  of  about  17  per  cent. 

All  the  mixed-gas  plants  listed  in  Table  2  except  Plants  13  and  15 
were  using  low-sulphur  eastern  coals  at  time  of  inspection.  In  Plants  13 
and  15,  mixtures  of  50  per  cent  eastern  and  50  per  cent  Illinois  coal  were 
used.  In  neither  of  these  plants  was  the  sulphur  content  of .  the  gas 
exceptionally  high,  but  of  course  the  40  per  cent  water-gas  made  in  both 
cases  tended  to  reduce  the  sulphur  in  the  mixed  gas.  It  is  evident  that  a 
decrease  of  17  per  cent  or  more  in  the  purifying  capacity  of  some  of  the 
mixed-gas  plants  given  would  be  a  serious  matter.  This  difficulty  might 
be  met  to  a  considerable  extent  by  more  attention  to  certain  details  of 
operation  and  selection  of  oxides,  as  has  been  previously  discussed. 

OBSERVED  RELATION  OF  OXIDE  VOLUME  TO  PURIFIER 

CAPACITY 

In  Tables  1  and  2  have  been  given  the  number  of  bushels  of  oxide  in 
use  in  each  plant  per  million  cubic  feet  of  gas  output  on  maximum  day. 
These  figures  have  not  been  used  in  computing  purifying  capacities  but  are 
given  partly  because  some  engineers  have  been  accustomed  to  rate  purifiers  in 
this  manner,  and  partly  to  show  the  effect  of  oxide  volume  on  purifying 
capacity  in  the  various  cases;  especially  in  connection  with  storage  capacity 
and  load  conditions.  In  Figure  3  is  plotted  the  relation  observed  between 
oxide  volume  and  maximum  hourly  purifying  capacity  in  the  various 
plants.  The  dotted  line  represents  1/10  bushel  of  oxide  per  cubic  foot  of 
gas  purified  during  maximum  hour,  and  is  drawn  in  for  convenience  of 
comparison.  The  water-gas  and  mixed-gas  plants  are  plotted  separately. 
It  is  evident  that  the  sulphur  content  of  the  gas  determines,  to  a  great 
extent,  the  position  of  the  curves,  and  therefore  any  specification  which 
calls  for  a  certain  number  of  bushels  of  oxide  per  1,000  cubic  feet  of  gas  to 
be  purified  in  a  given  time  can  at  best  be  only  incomplete.  It  is  interesting 
to  note  that  the  curves  for  mixed-gas  and  water-gas  plants,  while  quite 
close  together  for  the  smaller  plants,  appear  to  diverge  after  the  100,000 
cubic  feet  per  hour  production  is  passed.  Whether  this  would  hold  gen- 
erally for  the  larger  plants  cannot  be  stated  on  account  of  insufficient  data. 


Purifier  Operation 


43 


One  rather  remarkable  thing  is  shown  by  Tables  1  and  2,  namely, 
that  purifiers  may  be  worked  far  beyond  their  capacities  and  still  do  the 
work  required  of  them.  There  does  not  seem  to  be  any  definite  point,  at 
least  within  the  range  observed,  where  the  purifiers  actually  break  down 
suddenly  in  their  performance.     It  is  conceivable  that  gas  could  be  passed 


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BUSHELS  OF  OXIDE  IN   USE. 

Figure  3 — Observed  relation  of  oxide  volume  to  hourly  purification  capacity. 


through  a  series  of  boxes  containing  new  oxide  at  such  a  rate  that  hydrogen 
sulphide  could  be  detected  almost  immediately  at  the  outlet  of  the  system. 
In  testing  oxides  on  a  laboratory  scale,  this  condition  is  found  and  is  made 
use  of  to  determine  the  relative  rates  of  reaction  of  various  oxides. 

The  time  of  contact  called  for  by  the  Steere  formula  is  approximately 
6   minutes.      Many   comparatively  slow   oxides   will   not   give   a   test   for 


44  Gas  Purification  in  Medium  Size  Gas  Plants 

hydrogen  sulphide  immediately  on  a  gas  containing  100  grains  per  100 
cubic  feet  unless  the  time  of  contact  is  reduced  to  approximately  0.5 
minutes.  Other  more  rapid  oxides  will  absorb  all  of  the  hydrogen  sulphide 
from  gas  containing  100  grains  of  H2S  for  several  hours  with  this  time  of 
contact.  As  the  oxide  becomes  fouler,  however,  the  rate  of  absorption 
slows  down  and  eventually  some  hydrogen  sulphide  will  pass  by  the  box. 
This  will  happen  more  quickly,  other  things  being  equal,  in  a  purifying 
system  that  is  overloaded.  The  result  is  that  in  an  overloaded  plant,  other 
conditions  being  the  same,  more  frequent  changes  will  be  necessary  to  keep 
the  gas  clean.  When  the  frequency  of  changes  becomes  excessive  the 
operator  usually  has  one  of  the  following  choices;  namely,  to  enlarge  the 
purifying  equipment,  to  rearrange  existing  equipment,  to  improve  opera- 
tion by  increasing  the  absorption  per  bushel  through  more  complete  revivi- 
fication in  place,  in  some  of  the  ways  already  discussed,  or  to  find  a  more 
active  oxide.  Sometimes  in  the  more  extreme  cases,  only  the  first  alterna- 
tive will  prove  a  feasible,  permanent  remedy. 

REARRANGEMENT  OF  EQUIPMENT  TO  INCREASE 
CAPACITY 

As  just  mentioned,  the  capacity  of  a  purifying  installation  may  some- 
times be  materially  increased  by  minor  changes  whereby  the  existing  equip- 
ment can  be  used  in  a  more  advantageous  way,  or  the  existing  equipment 
may  be  enlarged,  without  changing  its  position  or  increasing  the  ground 
space  occupied.  Again,  internal  changes  are  possible  whereby  the  puri- 
fiers may  be  made  to  accommodate  a  greater  volume  of  oxide.  Frequently, 
where  the  existing  equipment  is  in  good  condition,  such  changes  may  be 
made  at  a  fraction  of  the  cost  of  an  entirely  new  installation.  Where 
additional  capacity  is  needed  it  wTould  often  pay  to  consider  (1)  whether 
the  best  possible  performance  is  being  obtained  from  the  present  equipment, 
and  (2)  whether  some  minor  changes  would  not  secure  sufficient  additional 
capacity  to  defer  the  installation  of  new  equipment  to  another  time.  Of 
course,  in  this  as  in  any  other  construction,  there  is  always  some  uncertainty 
as  to  the  relative  cost  of  installation  now  or  a  few  years  hence.  Some  of 
the  rearrangements  which  may  be  made  have  been  suggested  in  the 
detailed  descriptions  of  the  purifying  installations  inspected. 

In  the  first  place,  it  may  be  well  to  consider  whether  the  amount  of 
oxide  is  the  maximum  that  the  boxes  will  accommodate  or  whether  the 
layers  are  of  the  greatest  thickness  permissible.  In  several  instances  it  was 
found  that  the  available  space  for  oxide  in  the  boxes,  even  allowing  for  the 
trays  and  a  reasonable  amount  of  free  space,  was  considerably  greater  than 


Purifier  Operation  45 

would  correspond  to  the  number  of  bushels  said  to  have  been  purchased 
for  the  boxes. 

Again  the  amount  of  free  space  allowed  may  be  excessive.  In  some 
cases  the  trays  could  be  relocated  without  unduly  diminishing  the  free 
space  and  with  a  distinct  increase  in  capacity.  Take,  for  example,  a  4-box 
set  of  purifiers  arranged  for  reversible  flow,  each  box  being  15  feet  in 
diameter  and  12  feet  high,  containing  two  layers  of  oxide  each  \y2  feet  in 
depth.  The  hourly  capacity,  according  to  the  Steere  formula,  would  be 
61,845  cubic  feet,  if  the  gas  contained  200  grains  of  H2S  per  100  cubic 
feet.  Now,  assuming  that  the  depth  of  each  layer  could  be  increased  by  6 
inches,  the  capacity  would  become  66,262,  an  increase  of  capacity  of  14 
per  cent. 

If  the  space  available  for  oxide  is  being  utilized  to  the  fullest  extent, 
then  rearrangement  may  be  in  order.  As  has  been  suggested  in  several  cases, 
reversible  flow  may  help  considerably.  Let  us  assume  an  installation  of  three 
rectangular  boxes,  each  25  by  25  by  12  feet,  each  containing  two  5-foot 
layers  of  oxide  equipped  for  straight  flow  only,  and  with  a  given  sequence 
as  ABC,  BCA,  CAB.  Such  an  installation  purifying  gas  containing  200 
grains  of  H2S  per  100  cubic  feet  wTould  have  a  maximum  hourly  capacity, 
according  to  the  Steere  formula,  of  133,000  cubic  feet  of  gas. 

Let  us  assume  that  this  installation  is  overloaded  and  that  it  is 
desired  to  increase  its  capacity  about  35  per  cent.  This  increase  may  be 
accomplished  in  either  of  two  ways,  namely,  by  making  the  three  existing 
purifiers  perfectly  flexible  as  to  arrangement  and  reversibility  or  by  install- 
ing a  fourth  box  of  the  same  size  as  the  existing  boxes,  arranged  for  straight 
flow  only.  Leaving  out  of  consideration  for  the  present  the  difference  in 
operating  cost  which  would  be  in  favor  of  the  former  arrangement,  let  us 
consider  -the  probable  relative  costs  of  the  two  arrangements.  At  present 
prices,  the  cost  of  a  new  box  of  the  size  given,  arranged  for  straight  flow 
only,  would  probably  be  somewhere  between  $12,000  and  $16,000.  The 
installation  of  three  6-inch  reversing  valves,  together  with  alteration  of  the 
manifold  whereby  the  three  existing  boxes  could  be  made  entirely  flexible  as 
to  arrangement  and  reversibility,  would  cost  somewhere  between  $6,500  and 
$11,000,  depending  of  course  upon  the  amount  of  work  and  material  that 
would  be  required  and  the  amount  of  material  from  the  old  manifold  that 
could  be  applied  to  the  change.  It  is  evident  then  that  if  the  existing 
purifiers  were  in  good  condition  and  conveniently  arranged  for  operation, 
rearrangement  would  be  decidedly  cheaper  than  the  addition  of  another 
box  of  the  same  type.  The  economy  of  operation  would  also  be  distinctly 
in  favor  of  rearrangement.     A  number  of  installations  have  been  observed 


46  Gas  Purification  in  Medium  Size  Gas  Plants 

in  which    such    a    change    with    corresponding    increase    in    capacity    was 
apparently  feasible. 

In  a  few  cases  where  old  4-box  sets  with  center-seal  connections  are 
in  use,  a  change  in  the  connections  whereby  all  the  boxes  could  be  used  at 
one  time  would  be  advantageous. 

A  few  cases  have  also  been  observed  where  a  small  increase  in  capacity 
could  be  realized  by  putting  into  series  boxes  now  arranged  in  two  parallel 
groups,  but  in  most  of  the  cases  of  this  kind  observed,  such  a  change  would 
hardly  give  enough  increase  in  capacity  to  pay  unless  all  the  boxes  were 
made  reversible  and  arranged  so  as  to  be  used  in  any  desired  sequence.  In 
most  cases  where  two  parallel  groups  are  in  use,  one  of  the  groups  consists 
of  shallow  single-layer  boxes.  Frequently  these  shallow  boxes  are  of  con- 
siderable cross-sectional  area  and  where  substantial  foundations  exist  and 
plenty  of  head  room  is  available,  it  might  be  feasible  to  build  up  the  boxes 
to  double  height,  installing  an  additional  layer  of  oxide.  Such  a  rebuilt 
group  of  boxes,  if  arranged  for  reversible  flow  and  for  rotation  with  the 
boxes  of  the  deeper  group,  also  equipped  for  reversible  flow,  would  usually 
increase  the  capacity  of  the  system  very  materially.  For  example,  in  Plant 
No.  6  already  described,  such  a  reconstruction,  if  feasible,  would  increase 
the  capacity  by  about  66  per  cent. 

Of  course,  in  making  any  alterations  of  the  kinds  described  several 
things  have  to  be  considered.  It  would  obviously  be  unwise  to  go  to  con- 
siderable expense  to  alter  the  connections  of  or  reconstruct  boxes  which 
through  long  service  had  become  unsound.  And  it  might  be  unwise  to  pro- 
long the  use  of  boxes  so  arranged  that  the  cost  of  operation  was  excessively 
high  on  account  of  inconvenient  location  or  poor  facilities  for  handling 
oxide.  In  any  case  it  would  be  advisable  to  carefully  compare  the  cost  of 
rearrangement  or  reconstruction  with  the  cost  of  new  equipment  necessary 
to  give  an  increased  capacity  equivalent  to  that  expected  from  the  proposed 
change.  The  possibility  that  the  space  now  occupied  by  purifiers  especially 
when  in  substantial  buildings,  might  be  used  to  advantage  eventually  for 
some  other  purpose  should  also  be  considered.  Since  present  practice  is 
almost  unanimously  in  favor  of  outdoor  purifiers,  it  might  be  obviously 
unwise  to  perpetuate  the  use  of  valuable  buildings  for  this  purpose.  The 
economic  as  well  as  the  physical  features  of  such  a  change  need  considera- 
tion. In  all  cases  where  possible  changes  have  been  suggested  in  this 
paper,  it  is  to  be  understood  that  only  the  results  to  be  expected  from  such 
changes  have  been  considered.  The  considerations  just  named  and  physical 
conditions  existing  in  the  various  plants  might  make  the  suggested  altera- 
tion entirely  impracticable.  Each  case  would  have  to  be  considered  care- 
fully by  itself. 


Purifier  Operation  47 

RESULTS  OBTAINED  IN  PLANTS  INSPECTED 

As  will  be  seen  clearly  in  Tables  3  and  4,  purifier  capacity  alone  will 
not  necessarily  insure  good  purifying  results.  Some  of  the  best  results 
found  in  Illinois  plants  at  the  present  time  are  in  overloaded  plants. 

A  study  of  Tables  3  and  4  shows  how  difficult  or  impossible  it  is  to 
harmonize  the  results  actually  obtained  with  the  conditions  under  which 
they  were  obtained.  In  general,  one  cannot  but  be  impressed  by  the  differ- 
ence between  the  results  shown  and  those  generally  considered  as  typical 
of  the  best  practice.  Text  books  and  treatises  on  gas  manufacture  usually 
state  that  spent  oxide  will  contain  from  50  to  60  per  cent  of  sulphur, 
thereby  implying  that  this  degree  of  sulphiding  is  usually  attained  before 
the  oxide  is  discarded.  Yet  the  results  shown  in  the  tables  indicate  that  in 
the  average  Illinois  plant,  at  least,  not  nearly  the  usually  accepted  standard 
of  performance  is  actually  realized.  In  the  water-gas  plants,  excluding 
Plant  No.  5,  which  was  a  mixed-gas  plant  when  the  materal  was  fouled, 
the  average  percentage  of  sulphur  in  the  spent  oxide  was  only  21.7  per 
cent.  In  the  mixed-gas  plants,  on  the  other  hand,  including  Plant  No.  5, 
the  average  was  37.4  per  cent.  These  results  are  somewhat  lower  than 
those  reported  by  Mr.  Evans  in  the  paper  already  referred  to.  The  average 
absorption  in  the  water-gas  plants  studied  by  him  was  35  per  cent  sulphur 
and  in  the  coal-gas  plants  44  per  cent.  The  latter  figure  is,  we  understand, 
for  straight  coal-gas,  whereas  the  results  reported  by  us  in  Table  4  are  for 
mixed-gas  plants.  It  is  probable,  too,  that  the  plants  whose  results  are 
reported  by  Mr.  Evans  are  considerably  larger  in  size  than  the  plants  in 
Illinois  inspected  by  us,  and  therefore  the  conditions  were  probably  more 
favorable  for  good  results.  Even  so,  it  is  apparent  that  the  results  gener- 
ally obtained  fall  considerably  below  those  considered  as  good  standard 
practice. 

In  comparing  Tables  3  and  4  one  is  impressed  with  the  very  consid- 
erable difference  in  sulphur  absorption  in  the  two  gas-making  processes.  A 
number  of  reasons  for  the  difference  may  be  suggested.  Coal-gas  usually 
contains  considerably  more  sulphur  per  unit  volume  than  does  water-gas, 
and  according  to  the  law  of  mass  action,  the  greater  concentration  of  the 
H2S  in  coal-gas  increases  the  rate  of  absorption.  The  coal-gas  production 
is  also  more  uniform  and  the  peak  loads  on  the  purifiers  usually  represent  a 
greater  rate  of  water-gas  purification  rather  than  of  coal-gas,  so  the  average 
load  conditions  with  coal-gas  are  more  favorable.  The  ammonia  in  coal- 
gas,  of  which  traces  pass  through  the  purifiers,  may  also  assist  materially 
in  the  purifying  process  by  keeping  the  oxide  alkaline.  A  section  of  the 
Purification  Committee  of  the  American  Gas  Association  is  now  studying 
the  effect  of  this  factor. 


48 


Gas  Purification  in  Medium  Size  Gas  Plants 


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50  Gas  Purification  in  Medium  Size  Gas  Plants 

In  the  columns  of  Tables  3  and  4  marked  "Total  gas  purified  per 
bushel — M  cu.  ft.",  the  assumption  has  been  made  that  the  H2S  content 
of  the  gas  was  constant  and  the  same  as  at  the  time  of  inspection.  The 
figures  reported  are  probably  quite  as  accurate  as  the  results  reported  in  the 
purification  records  of  most  plants,  but  owing  to  the  uncertainty  of  the 
assumption  on  which  they  are  based,  no  claims  to  great  accuracy  are  made 
for  them. 

Causes  of  Low  Efficiencies 
overload 

The  cause  of  the  discrepancy  between  standard  purification  practice 
and  the  actual  results  observed  in  Illinois  plants  is  a  complex  one.  It  is 
probably  due  to  some  extent  to  overload.  That  an  overloaded  condition 
of  the  purifying  equipment  can  result  in  low-sulphur  absorption  is  evident 
from  a  consideration  of  the  effect  of  overload  on  operation.  Where  the 
gas  has  to  be  passed  through  the  boxes  at  an  excessive  rate,  although  the 
oxide  may  at  first  completely  purify  it,  the  condition  where  it  cannot  com- 
pletely remove  the  sulphur  is  reached  sooner  than  would  be  the  case  if  the 
rate  of  gas  flow  were  slower.  The  time  between  box  changes  will  there- 
fore be  shorter  with  the  faster  rates. 

The  complete  absorption  of  hydrogen  sulphide  requires  a  measurable 
time  of  contact  between  gas  and  oxide.  When  the  material  is  fresh  a 
shorter  time  of  contact  seems  adequate,  but  as  the  surface  of  the  oxide 
becomes  sulphided,  a  longer  time  of  contact  seems  to  be  necessary  for  com- 
plete absorption  of  hydrogen  sulphide  by  the  inner  particles  of  oxide.  With 
very  rapid  rates  of  gas  flow,  the  absorption  seems  to  be  at  first  largely 
superficial,  and  after  the  superficial  capacity  of  the  material  has  been 
utilized,  longer  contact  seems  necessary  for  complete  absorption  of  H2S. 

The  absorption  of  sulphur  per  change  will  therefore  be  less  with  an 
overloaded  purifying  system,  and  to  get  the  same  absorption  an  oxide  will 
have  to  be  handled  more  times  than  would  be  necessary  were  the  flow 
slower.  Each  time  an  oxide  is  handled  it  becomes  finer  from  breakage,  and 
with  frequent  handling  the  oxide  soon  becomes  so  fine  as  to  favor  packing 
and  back  pressure.  Conditions  which  lead  to  overload  of  the  purifiers  also 
usually  favor  overload  of  the  other  gas-cleaning  equipment,  so  that  more 
tar  is  likely  to  be  carried  into  the  boxes,  other  conditions  being  the  same, 
when  the  purifiers  are  worked  beyond  their  capacities.  Therefore,  more 
rapid  tarring  and  excessive  breakage  of  oxide  in  an  overloaded  plant  are 
likely  to  result  in  depreciation  and  discard  of  the  purifying  material  sooner 
than  in  a  plant  operated  at  normal  capacity. 


Purifier  Operation 


51 


We  note,  however,  that  some  of  the  overloaded  plants  excel  other 
similar  underloaded  plants  in  absorption  realized,  so  this  does  not  appear 
to  be  the  only  or  perhaps  the  most  serious  cause. 

tar  in  the  gas 

Tar  in  the  gas  is  also  a  contributory  cause,  but  it  is  difficult  to  say  in 
a  particular  case  to  just  what  extent  efficiency  was  reduced  by  it,  for  some 
of  the  spent  oxides  collected  that  are  high  in  tar  are  also  high  in  sulphur. 
This  would  lead  one  to  think  offhand  either  that  tar  has  little  effect  on 
sulphur  absorption  or  that  most  of  the  sulphur  absorption  had  taken  place 
before  the  oxide  was  tarred.  In  order  to  determine  roughly  the  effect  of 
tar  in  the  oxide  upon  efficiency  of  its  absorption  of  hydrogen  sulphide,  an 
experiment  was  carried  out,  the  results  of  which  are  shown  in  Figure  4 


0, 

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Figure  4.     Effect  of  tar  on  the  absorption  capacity  of  an  oxide  for  hydrogen 

sulphide,  first  fouling. 

The  absorption  capacity  of  a  particular  oxide  for  pure  hydrogen  sulphide 
was  determined  by  the  Kunberger  method.  Portions  of  the  same  oxide 
were  then  treated  with  various  percentages  of  tar,  and  their  absorption 
capacities  after  treatment  were  similarly  determined.  The  tar  was  applied 
by  making  up  a  solution  of  water-gas  tar  in  pure  carbon  bisulphide.  Each 
sample  of  oxide  was  treated  with  an  amount  of  this  tar  solution  corre- 
sponding to  the  required  percentage  of  tar.  The  tar  solution  and  oxide 
were  thoroughly  mixed;  then  the  carbon  bisulphide  was  removed  by  evap- 
oration in  the  air.  The  tar  was  left  deposited  on  the  oxide,  which  was 
then  subjected  to  the  fouling  test. 

While   the   conditions  of   the  test   and   the   results  obtained   are   not 
strictly  comparable  with  practical  conditions,  they  indicate  something  con- 


52 


Gas  Purification  in  Medium  Size  Gas  Plants 


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Purifier  Operation  53 

cerning  the  effect  of  tar  in  coating  oxide  and  decreasing  its  absorption 
capacity.  Of  course,  the  ability  of  the  material  to  revivify  is  also  affected, 
so  that  tar  is  a  disadvantageous  thing  to  have  present  at  any  and  all  times. 

It  was  thought  at  first  that  the  reason  for  the  comparatively  high 
sulphur  percentages  in  some  rather  tarry  oxides  might  be  due  to  the  tar- 
extracting  action  of  a  portion  of  the  oxide  batch,  permitting  the  remainder 
of  the  batch  to  foul  more  completely  on  tar-free  gas.  It  was  expected  that 
analyses  of  oxide  from  different  levels  in  an  upward-flow  box  would  show 
a  concentration  of  tar  in  the  lower  part  of  the  box.  Samples  of  oxide  were 
therefore  taken  in  two  plants,  one  water-gas  and  the  other  coal-gas,  at 
various  levels.  Each  sample  was  collected  from  several  points  at  the  same 
level,  to  insure  a  representative  sampling.  Analyses  of  the  samples  and 
the  levels  where  taken  are  shown  in  Table  5.  It  is  surprising  to  note  that 
the  t.ar  is  not  concentrated  at  the  bottom.  In  the  water-gas  plant  the 
greatest  percentage  of  tar  was  found  in  the  bottom  of  the  upper  layers, 
while  in  the  coal-gas  box,  the  top  of  the  batch  contained  the  most  tar. 
Likewise,  there  is  no  apparent  relation  between  the  percentages  of  tar  and 
sulphur  in  these  batches.  The  highest  percentage  of  sulphur  in  each  case 
is  coincident  neither  with  the  lowest,  nor  with  the  highest  percentage  of 
tar.  One  naturally  concludes  that  each  batch  must  have  absorbed  the  larger 
part  of  its  sulphur  in  each  case  before  the  tar  was  present  to  any  great 
extent.  The  concentration  of  tar  in  the  upper  part  of  the  batch  may  be 
caused  by  tar  condensing  out  of  the  gas,  due  to  the  cooling  action  of  the 
purifier  box  cover,  and  dropping  down  into  the  oxide.  Of  course,  in  prac- 
tice the  accumulation  of  tar  is  relatively  slow,  and  since  the  rate  of  sulphur 
absorption  in  a  given  part  of  an  oxide  batch  is  slowing  down  as  the  cen- 
centration  of  iron  sulphide  increases,  it  is  difficult  to  determine  in  a  partic- 
ular case  to  just  what  extent  the  accumulation  of  tar  is  affecting  the  per- 
formance. 

The  means  of  removing  tar  differs  considerably  in  different  plants. 
Table  6  shows  the  gas-condensing  and  scrubbing  equipment  in  use  in  the 
different  plants.  For  convenience,  the  final  tar-extracting  apparatus  in 
each  case  is  printed  in  italics.  It  will  be  noted  that  the  shavings  scrubber 
appears  to  be  the  favorite  tar-extracting  equipment  in  water-gas  plants, 
though  the  P.  &  A.  tar  extractor  and  the  bubble  washer  are  preferred  in  a 
few  plants.  The  writers  of  this  bulletin  have  had  no  opportunity  to  study 
the  relative  merits  of  these  various  types  of  apparatus  under  conditions 
which  were  comparable.  As  will  be  noted  in  the  table,  the  amount  of  tar 
in  the  gas  at  the  inlet  of  the  purifiers  varies  considerably  even  with  the 
same  type  of  tar  extractor,  and  it  seems  to  be  possible,  under  favorable  con- 
ditions, to  obtain  practically  complete  extraction  with  any  one  of  these 


54 


Gas  Purification  in  Medium  Size  Gas  Plants 


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Purifier  Operation  55 

types  of  apparatus.  At  the  time  when  the  tar  tests  were  taken,  from  which 
the  pounds  of  tar  per  million,  as  given  in  Tables  3  and  4,  were  computed, 
the  temperature  was  very  high.  Hence,  the  results  represent  probably  the 
most  unfavorable  conditions.  The  tar-extracting  apparatus  of  many  of  the 
plants  was  overloaded  at  that  time.  The  overload  was  due  in  some  cases 
to  inadequate  apparatus  and  in  others  to  the  temperature  of  the  cooling 
water  in  the  various  units  being  unavoidably  high.  It  will  be  noted  that 
where  the  temperature  at  the  inlet  of  the  purifiers  is  above  100°F.  the 
amount  of  tar  remaining  in  the  gas  is  usually  excessive,  regardless  of  the 
apparatus,  though  this  is  not  an  invariable  rule.  The  water-gas  plants 
show  higher  average  temperature  at  the  purifiers  than  do  the  mixed-gas 
plants,  and  a  comparison  of  the  tar  contents  of  the  gases  shows  the  average 
water-gas  to  contain  about  twice  as  much  tar  per  million  cubic  feet  as  does 
the  average  mixed-gas.  Where  shavings  scrubbers  are  employed,  there 
seems  to  be  in  many  cases  no  regular  routine  observed  in  regard  to  changing 
the  shavings,  and  the  indications  are  that  in  some  cases  they  are  allowed  to 
remain  unchanged  too  long.  The  frequency  of  changes  would  of  course 
depend  upon  the  tar  content  of  the  gas  at  this  point  in  the  system,  and  this 
in  turn  would  be  affected  by  the  operation  of  the  previous  equipment.  In 
one  plant  at  least,  notably  Plant  No.  7,  a  regular  routine  is  observed, 
each  bushel  of  shavings  cleaning  approximately  100,000  cubic  feet  of  gas. 
It  will  be  observed  that  the  tar  entering  the  boxes  amounted  to  only  9.7 
pounds  per  million  cubic  feet  of  gas.  That  this  efficiency  had  not  been 
realized  at  all  times,  however,  is  indicated  by  the  previous  column,  which 
shows  that  spent  oxide  from  this  plant  contained  a  rather  high  tar  content 
of  4.4  pounds  per  bushel.  On  the  other  hand,  some  plants  which  at  the 
time  of  the  test  showed  high  tar,  give  indications  of  better  past  perform- 
ance, as  judged  by  the  analyses  of  spent  oxide.  It  seems  as  though  regular 
tests  of  the  gas  for  tar  by  a  tar  camera  or  other  device  would  be  worth 
while  in  the  better  purifying  results  effected.  It  is,  however,  by  no  means 
common  to  find  plants  which  possess  the  necessary  apparatus  but  seldom 
use  it.  This  subject  deserves  more  consideration  than  is  ordinarily  given 
to  it.  It  is  probably  safe  to  say,  in  spite  of  the  difficulty  in  demonstrating 
to  a  certainty,  that  tar  interferes  with  purification  efficiency  more  than  any 
other  single  cause. 

METHODS  OF   REVIVIFICATION   IN    USE 

Inspection  of  Tables  3  and  4  is  likely  to  surprise  advocates  of  revivi- 
fication in  place.  It  will  be  noted  that  whereas  most  of  the  straight  water- 
gas  plants  introduce  from  1  to  2  per  cent  of  air  into  the  gas  prior  to  purifi- 
cation, the  mixed-gas  plants  with  one  exception  resort  to  the  old  method  of 


56  Gas  Purification  in  Medium  Size  Gas  Plants 

revivifying  oxide  in  the  open  air.  Revivification  in  place  has  been  in  use 
for  many  years.  Its  theoretical  advantages,  especially  under  the  now 
almost  universal  heating-value  standard,  are  almost  unquestionable.  Why 
then  is  it  not  more  universally  used?  One  coal-gas  operator,  when  ques- 
tioned in  regard  to  this  matter,  stated  that  his  retort  settings  leaked  so  that 
enough  air  was  admitted.  Whether  the  leakage  was  really  air  or  furnace 
gases  was  not  demonstrable  by  any  tests  made  in  his  plant.  Other  operators 
stated  that  so  light  a  seal  was  carried  on  the  hydraulic  main  that  air  was 
drawn  in  every  time  a  retort  was  opened.  While  it  is  perhaps  true  that 
there  is  oftentimes  considerable  leakage  of  air  or  gas  into  the  gas  system, 
this  is  a  most  uncertain  and  doubtful  way  of  revivifying  in  place,  and 
possesses  other  disadvantages  as  well.  When  the  retorts  are  leaky,  it  may 
be  inadvisable  for  the  time  being  to  dilute  the  gas  any  further,  but  it  would 
hardly  seem  reasonable  that  this  uncertain  method  of  introducing  air  is  in 
vogue  in  a  majority  of  the  coal-gas  plants.  Other  objections  to  revivifica- 
tion in  place  are  frequently  advanced,  including  the  additional  cost  of 
enrichment  to  make  up  for  the  inert  nitrogen  introduced  into  the  gas. 
Another  objection  frequently  advanced  is  that  the  oxide  has  a  tendency  to 
cake  very  hard  when  revivification  in  place  is  tried.  It  is  probably  true 
that  where  an  attempt  is  made  to  completely  foul  an  oxide  to  its  capacity 
with  one  handling,  the  caking  will  be  pronounced,  but  it  is  believed  that  an 
attempt  to  carry  revivification  to  this  extreme  is  seldom  practiced.  Where 
caking  has  been  found  to  occur  with  revivification  in  place,  study  should 
be  given  to  the  subject  with  a  view  to  relieving  this  condition  without 
abandoning  revivification  in  place  altogether.  Some  operators  state  that  a 
lack  of  a  proper  amount  of  moisture  in  the  gas  is  a  fruitful  cause  of  caking. 
An  excessive  loading  of  the  material  with  iron,  whereby  the  weight  per 
bushel  is  greatly  increased,  might  cause  trouble,  especially  in  deep  beds  of 
oxide.  Under  such  conditions,  consideration  might  well  be  given  to  the 
question  whether  the  extra  iron  really  pays  for  itself  in  amount  of  absorp- 
tion obtained.  It  may  be  that  a  lighter  oxide  capable  of  revivifying  rapidly 
might  realize  a  considerably  greater  absorption  in  the  long  run.  The  car- 
rier used  may  also  be  a  factor  in  the  case.  If  the  carrier  consists  of  planer 
chips,  then  consideration  might  well  be  given  to  the  size  of  the  chips;  per- 
haps the  particular  size  used  packs  too  much,  giving  a  better  opportunity 
for  the  sulphur  deposited  during  revivification  to  cement  the  particles  of 
oxide  together.  The  nature  of  the  carrier  is  one  of  the  objections  which 
has  been  presented  to  the  use  of  blast-furnace  slag  as  a  carrier  for  oxide.  It 
is  claimed  by  some  that  such  oxide  is  especially  likely  to  mat  together, 
giving  a  product  which  is  heavy  and  difficult  to  handle.  It  may  be  that 
this  type  of  material  might  be  objectionable  in  some  cases. 


Purifier  Operation  57 

Another  reason  why  revivification  in  place  is  neglected  to  such  an 
extent  may  be  that  it  has  been  tried  and  that  results  do  not  bear  out  the 
performance  expected.  In  such  cases,  it  would  be  well  to  ascertain  to  what 
extent  the  introduction  of  air  really  affected  the  matter,  and  whether,  as  ? 
matter  of  fact,  the  results  might  not  have  been  due  to  other  conditions. 

Timidity  about  introducing  air  into  the  gas  may  be  a  cause  of 
hesitancy  on  the  part  of  some  operators.  It  may  be  feared  that  the  quality 
of  the  gas  will  be  unduly  affected,  and  that  the  general  public  may  learn  of 
the  practice  and  misunderstanding  the  motive  behind  it,  raise  a  clamor. 
There  seems  little  real  foundation,  however,  for  such  forebodings,  as  is 
evidenced  by  the  fact  that  some  of  the  best-operated  plants  have  used  the 
method  for  years  without  serious  protest. 

Revivification  in  the  off-box  was  practiced  in  only  one  of  the  plants 
inspected,  namely,  in  Plant  No.  16.  The  cost  of  purification  in  this  plant 
was  lower  per  unit  of  hydrogen  sulphide  in  the  gas  than  in  any  other 
plant.  A  brief  description  of  the  method  as  applied  in  this  plant  may  be 
helpful. 

A  box  is  blown  shortly  after  it  begins  to  show  foul,  as  tested  by  lead 
acetate  paper.  At  that  time  the  box  is  usually  removing  about  60  per  cent 
of  the  H2S  in  the  gas  (this  would,  of  course,  depend  a  great  deal  upon  the 
nature  of  the  oxide).  The  rate  of  circulation  of  air  through  the  box  is  about 
2,000  cubic  feet  per  minute  (about  double  the  maximum  rate  of  gas  flow). 
The  air  is  blown  through  the  box  opposite  to  the  direction  of  gas  flow, 
just  previous  to  the  time  of  turning  off  the  box.  The  air  is  not  cooled 
during  circulation,  nor  is  any  steam  introduced  with  the  air.  This  was 
tried  at  one  time  but  resulted  in  warping  the  grids.  Temperature  observa- 
tions are  taken  on  the  air  as  it  leaves  the  box.  The  blowing  is  continued 
until  a  20°F.  rise  in  temperature  is  observed.  At  this  point  the  fresh  air 
supply  is  cut  off  and  the  deoxygenated  air  circulation  is  continued  until  a 
decided  drop  in  temperature  is  observed.  Some  trouble  from  channelling 
and  local  heating  has  been  experienced.  The  oxide  is  usually  removed 
from  the  box  only  twice  before  final  discard.  The  oxide  has  a  tendency  to 
cake  harder  with  this  method  than  without.  Even  with  such  difficulties 
as  are  encountered,  the  superintendent  in  charge  considers  it  the  most 
economical  means  of  revivification.  As  has  been  mentioned  in  the  detailed 
description  of  the  purifying  equipment  of  this  plant  previously  given,  the 
boxes  in  which  this  method  of  revivification  is  used  are  arranged  for 
reversible  flow,  but  there  is  only  one  sequence  of  boxes  possible.  The 
superintendent  of  the  plant  expresses  the  opinion  that  were  the  sequence  of 
the  boxes  capable  of  alteration,  together  with  the  reversible  flow,  it  would  be 
possible  to  blow  the  boxes  before  they  became  very  foul  and  that  chan- 


58  Gas  Purification  in  Medium  Size  Gas  Plants 

nelling  and  local  heating  troubles  as  well  as  caking  of  the  oxide  would  be 
largely  avoided. 

While  this  opinion  as  expressed  may  be  conjecture  only,  it  accords 
quite  well  with  the  conclusions  drawn  by  O.  B.  Evans  in  his  paper, 
entitled  "Revivification  in  place",  already  referred  to.  It  seems  likely  that 
the  temperature  rise  during  revivification  dries  out  the  oxide  to  a  consider- 
able extent  and  thereby  favors  caking.  Were  a  surface  condenser  or  other 
means  used  whereby  the  air  would  become  thoroughly  saturated  with 
moisture  during  circulation,  it  seems  likely  that  the  caking  would  be 
diminished,  since  the  water-saturated  air  would  have  a  tendency  to  keep 
the  oxide  moist. 

This  method  variously  modified  has  been  used  for  years  by  several 
large  gas  companies  and  from  the  experiences  in  the  plant  mentioned  above 
it  would  seem  to  merit  more  attention  from  Illinois  operators  than  it  has 
received. 

Even  the  practice  of  revivifying  oxide  in  the  open  air  could  be  im- 
proved in  a  number  of  cases.  It  is  not  uncommon  for  a  considerable  por- 
tion of  the  batch  to  overheat  during  revivification  in  the  open,  which,  of 
course,  has  a  detrimental  effect  on  the  sulphur-absorbing  capacity  of  the 
material. 

Revivification  in  place  seems  by  far  the  preferable  method.  Its 
advantages  might  well  be  studied  by  those  who  are  not  now  using  it.  The 
writers  can  do  no  better  than  to  refer  to  O.  B.  Evans'  paper,  "Revivifica- 
tion in  place",  presented  to  the  American  Gas  Association  at  their 
October;  1919,  meeting.  It  is  believed  that  the  Association  can  supply 
copies  of  this  paper  for  a  few  cents  each. 

LACK  OF  TESTS  AND  RECORDS 

In  every  plant  inspected  by  the  writer,  inquiries  were  made  relative 
lo  the  chemical  control  of  purification  practiced  and  the  nature  of  the 
purifying  records  kept.  It  was  rather  surprising  to  learn  how  few  operators 
really  do  make  any  systematic  effort  to  study  their  purification  problem 
from  a  technical  standpoint.  In  a  majority  of  plants  the  lead  acetate  paper 
test  was  the  only  sulphur  test  made  aside  from  the  total  sulphur  test  on  the 
purified  gas,  required  by  the  Public  Utilities  Commission.  About  half  of 
the  plants  visited  possessed  a  Tutwiler  apparatus,  but  in  several  cases  the 
instrument  was  broken,  or  the  stop-cocks  were  stuck,  and  in  only  one  case 
which  we  recall  were  systematic  tests  made  every  day.  The  operators 
usually  had  only  a  hazy  idea  as  to  what  their  variouse  purifiers  were  doing. 

The  purifying  records  were  in  many  cases  fragmentary  and  in  a  few 
cases  kept  on  loose  sheets  of  paper.     In  a  few  cases  a  separate  oxide  batch 


Purifier  Operation  59 

record  was  kept.  The  usual  record  showed  the  amount  of  gas  passed  by 
the  first  box  of  each  sequence,  between  changes,  and  usually  the  purifica- 
tion of  all  this  gas  was  credited  to  the  first  box.  The  sulphur  content  of 
the  unpurified  gas  often  varied  considerably  from  day  to  day,  but  this  was 
not  taken  into  account  in  crediting  a  particular  batch.  The  assumptions 
were  really  made,  so  far  as  the  records  went,  that  the  H2S  content  of  the 
gas  was  uniform,  and  that  the  various  batches  as  they  came  into  first  posi- 
tion would  average  up:  viz.,  the  first  batch  would  receive  credit  for  more 
than  actual  purification  to  offset  the  unearned  credit  given  to  other  batches 
while  they  were  in  first  position.  In  some  plants  analyses  of  spent  oxides  had 
been  made,  chiefly  to  value  the  oxides  for  cyanide  recovery,  and  in  such  cases 
the  sulphur  per  bushel  was  reported,  but  except  in  such  cases,  the  records 
showed  no  figures  from  which  the  total  sulphur  absorption  of  a  batch  per 
bushel  could  be  computed  with  any  degree  of  accuracy.  In  the  one  plant 
where  tests  were  regularly  made  and  records  kept  of  batch  performance, 
it  is  interesting  to  note  that  the  total  cost  of  purification  per  100  grains  of 
H2S  in  the  gas  to  be  purified  is  lower  than  in  any  other  plant.  In  view 
of  the  small  expenditure  of  time  and  equipment  to  gain  this  information  it 
seems  worthy  of  more  consideration. 

Cost  of  Purification 

The  ultimate  criterion  by  which  the  gas  operator  judges  purification 
performance  in  his  plant  is  cost  per  1,000  cubic  feet  of  gas  purified.  So 
long  as  the  sulphur  content  of  the  unpurified  gas  remains  low  and  purifi- 
cation does  not  present  an  undue  amount  of  difficulty  from  an  operating 
standpoint,  the  operator  is  likely  to  be  satisfied,  provided  the  costs  are 
reasonably  low.  It  is  when  the  necessity  or  advantage  of  using  fuels  of 
higher  sulphur  content  presents  itself  or  when  the  costs  begin  to  mount 
that  the  average  operator  begins  to  inquire  into  purification  efficiencies. 

The  cost  figures  given  in  Tables  3  and  4  are  averages  for  1919.  Even 
a  casual  inspection  of  these  figures  in  connection  with  the  load  and  other 
conditions  in  the  various  plants  shows  that  there  is  no  direct  relation  be- 
tween purifier  load  and  purifying  costs  so  far  as  can  be  ascertained  from 
a  comparison  of  different  plants.  Operating  methods,  size  of  plant,  sul- 
phur content  of  the  gas,  cost  of  labor,  facilities  for  handling  oxide  to  and 
from  the  boxes,  cost  of  purifying  materials  (including  freight),  all  have 
much  influence  on  purifying  costs.  The  only  way  to  determine  the  effect 
of  a  single  factor  as  purifier  load,  would  be  to  make  an  extensive  compari- 
son of  costs  before  and  after  an  extension  of  the  purifying  system,  care 
being  taken  to  keep  all  other  factors  constant.  The  writers  have  no  data 
from  which  any  conclusions  can  be  drawn  relative  to  this  point. 


60  Gas  Purification  in  Medium  Size  Gas  Plants 

Inspection  of  Tables  3  and  4  indicates  that  the  average  total  purifica- 
tion cost  in  the  mixed-gas  plants  (average  about  25  per  cent  water-gas)  is 
about  one-third  higher  than  the  average  cost  for  water-gas  plants,  but  the 
sulphur  content  of  the  mixed-gas  averages  considerably  higher  than  that  of 
the  water-gas  plants.  Calculated  to  a  basis  of  sulphur  absorbed  per 
bushel  of  oxide,  the  cost  of  mixed-gas  purification  would  probably  be  con- 
siderably lower  than  that  of  water-gas  purification.  The  gas  operator 
figures  all  his  operating  costs,  including  purification,  to  a  basis  of  1,000 
cubic  feet.  While  this  is  necessary  in  order  to  be  in  harmony  with  his 
other  operating  costs,  purification  results,  especially  in  different  plants, 
could  be  better  compared  on  the  basis  of  cost  per  pound  of  sulphur 
(or  H2S)  absorbed  per  bushel  of  oxide.  This  could  readily  be  determined, 
from  the  purifying  cost  per  thousand  if  the  sulphur  content  of  the  oxide 
and  the  average  H2S  content  of  the  gas  were  known.  Regular  tests  and  a 
form  of  record  such  as  is  suggested  in  Appendix  B  of  this  paper  would 
furnish  the  necessary  information. 

The  operating  cost  is  of  course  only  a  part  of  the  cost  of  purification. 
The  capital  charges  on  the  equipment  are  just  as  truly  a  part  of  the  cost 
and  in  some  cases  they  may  approach  the  cost  of  operation.  In  making 
extensions  to  existing  equipment,  if  greater  efficiency  of  operation  is  the 
main  thing  sought,  it  is  well  to  inquire  whether  the  total  purification  cost 
will  be  decreased  by  the  added  equipment.  A  concrete  example  will  serve 
to  illustrate  this.  A  certain  plant  in  Illinois  has  a  total  annual  gas  output 
of  about  550  million  cubic  feet  of  gas.  Its  present  purifying  capacity  is 
40,000  cubic  feet  per  hour.  The  maximum  hourly  production  is  about 
110,000  cubic  feet.  The  purifying  apparatus  is  therefore  operating  at 
about  275  per  cent  of  its  rated  capacity.  In  spite  of  this  heavy  overload, 
the  total  purifying  operation  cost  for  1919  was  reported  as  only  0.7  cents 
per  1,000  cubic  feet  of  gas  purified.  Great  difficulty  has  been  experienced 
in  completely  purifying  the  gas  before  distribution  and  therefore  the  man- 
agement has  let  a  contract  for  a  new  purifier  which  together  with  the 
necessary  connections  to  tie  it  in  with  the  existing  boxes  will  cost  about 
$18,000.  The  capital  charges  on  the  old  equipment  are  not  known  and 
need  not  be  considered,  since  these  charges  will  remain  as  before,  even 
after  the  new  box  is  installed.  Capital  charges  on  the  new  investment  will 
be  about  14  per  cent  per  annum,  or  $2,520.  This  amount  will  correspond 
to  about  0.46  cents  per  1,000  cubic  feet  of  gas  purified,  which  is  consider- 
ably more  than  half  of  the  total  purifying  operating  cost  per  1,000  for  last 
year.  It  is  obvious,  therefore,  that  unless  the  purifying  operating  cost  can 
be  reduced  to  about  0.24  cents  per  thousand  there  will  be  no  immediate 
financial   gain   from   the   new   installation,   though   the   company  will   be 


Purifier  Operation  61 

relieved  from  much  of  the  anxiety  under  which  it  has  been  placed.  This, 
while  perhaps  not  expressible  in  dollars  and  cents,  is  worth  considering. 
The  purifying  operating  cost  for  1919,  namely,  0.7  cents  per  thousand,  is 
lower  than  in  a  majority  of  plants  of  the  same  size,  and  it  is  possible  that 
it  is  not  absolutely  accurate.  There  is  often  considerable  difficulty,  for 
example,  in  equitably  distributing  the  cost  of  oxide  purchased  over  a  proper 
period  of  time.  That  such  an  e.rror  might  have  been  present  in  this  case 
is  indicated  by  the  fact  that  the  purifying  materials  cost  was  only  0.2  cents 
per  thousand,  about  one-fourth  that  in  plants  obtaining  similar  efficiencies. 
The  labor  cost,  0.5  cents  per  thousand,  was  somewhat  higher  than  the 
average,  as  might  be  expected  from  the  exceptionally  overloaded  conditions 
prevailing.  The  total  purifying  operation  cost  was  therefore  probably 
nearer  1.3  cents  per  thousand.  The  cost  of  removing,  revivifying,  and  re- 
placing the  oxide  was  probably  at  least  5  cents  per  bushel.  This  would  cor- 
respond to  about  25  changes  of  oxide  per  year,  or  one  change  every  two 
weeks.  The  gas  purified  per  change  was  only  about  10,577,000  cubic  feet, 
or  4,800  cubic  feet  per  bushel.  The  sulphur  absorptionwas  therefore  only 
about  0.65  pounds  per  bushel  per  change.  If  the  analysis  of  the  spent  ox- 
ide from  this  plant  is  typical  of  the  perforance  obtained,  each  batch  prob- 
ably had  to  be  handled  about  eight  times  to  get  an  absorption  of  5.3  pounds 
of  sulphur  per  bushel,  and  the  oxide  was  discarded  when  it  had  taken  up 
only  about  one-third  as  much  sulphur  as  is  usually  absorbed  in  water-gas 
practice. 

The  new  installation  will  increase  the  computed  capacity  from  40,000 
cubic  feet  to  about  117,000  cubic  feet  per  hour.  The  arrangement  will  be 
such  that  there  will  be  complete  flexibility  as  to  sequence  of  boxes  and 
direction  of  flow.  Other  projected  improvements  should  result  in  bring- 
ing gas  containing  much  less  tar  than  heretofore,  to  the  purifiers,  so  that 
it  will  probably  be  possible  to  operate  with  much  less  handling  of  oxide. 
Assuming  that  each  batch  could  be  fouled  to  35  per  cent  sulphur  with  two 
changes,  this  would  amount,  with  the  present  sulphur  content  of  the  gas,  to 
a  removal  and  a  replacement  of  about  7,800  bushels  per  year.  At  5  cents 
per  bushel  for  handling,  the  annual  labor  cost  would  amount  to  $390,  or 
.07  cents  per  thousand.  An  average  of  about  3,900  bushels  of  oxide  would 
be  discarded  per  year.  Assuming  the  cost  of  40  cents  per  bushel  of  oxide 
f.  o.  b.  the  gas  plant,  this  would  amount  to  $1,560  per  year  or  0.29  cents 
per  thousand.  The  total  purifying  operation  cost  then  would  appear  to  be 
about  0.36  cents  per  thousand,  or  about  0.94  cents  per  thousand  less  than 
the  probable  cost  of  purification  for  1919,  or  0.34  cents  less  than  the 
reported  cost.  The  total  cost  including  capital  charges  will  be  about  0.82 
cents  per  thousand.     It  seems  then  that  after  adding  capital  charges,  even 


62  Gas  Purification  in  Medium  Size  Gas  Plants 

if  operation  as  good  as  that  assumed  above  is  obtained,  the  company  can 
hardly  hope  to  purify  the  gas  as  cheaply  as  they  reported  last  year.  How- 
ever, even  after  adding  to  the  operation  costs,  the  capital  charges  due  to 
the  new  installation,  they  will  save  about  0.48  cents  per  thousand,  as  com- 
pared with  the  probable  actual  costs  of  operation  last  year.  In  arriving  at 
these  conclusions,  some  assumptions  have  been  made  which  may  or  may  not 
be  accurate,  but  the  example  serves  to  illustrate  the  nature  of  the  problem 
which  faces  the  operator  who  contemplates  an  extension  to  his  purifying 
system. 

As  has  been  previously  mentioned,  while  extensions  are  undoubtedly 
needed  in  a  number  of  cases,  good  operation  may  go  a  long  way  to  offset 
insufficient  equipment.  On  the  other  hand,  cases  have  been  observed  where 
it  seemed  that  a  much  over-size  installation  gave  such  a  sense  of  security 
to  the  operator  that  his  watchfulness  was  relaxed  and  he  lost  the  oppor- 
tunities for  operating  economy  which  his  plant  facilities  would  have  per- 
mitted. 

CONCLUSION 

Though  considerable  improvement  in  purifying  conditions  in  indi- 
vidual cases  can  be  expected  by  application  of  the  various  suggestions 
which  have  been  made,  the  process  itself  has  inherent  faults  which  cannot 
be  avoided.  It  is  cumbersome,  requires  heavy  investment,  and  since  it 
involves  the  handling  of  solid  material  must  be  costly  in  operation.  Many 
gas  engineers  have  looked  forward  to  the  day  when  some  liquid  purification 
process  or  some  other  process  of  similar  theoretical  merit  would  supplant 
oxide  purification.  Many  such  processes  have  been  suggested  and  more 
than  one  has  had  promise  of  practicability.  The  fact  remains,  however, 
that  millions  of  dollars  are  now  invested  in  existing  equipment  for  purify- 
ing gas  by  iron  oxide.  Oxide  purifiers  are  continually  being  built  to  sup- 
plement those  now  in  use,  and  even  were  a  more  economical  process 
worked  out  for  purifying  gas,  the  replacement  would  necessarily  be  slow, 
since  not  all  companies  would  be  able  to  scrap  existing  equipment  at  once. 
Any  study  which  will  improve  operation  with  present  equipment  is  there- 
fore worth  while.  It  seems  as  though  the  greatest  promise  of  material 
advancement  in  oxide  purification  generally  lies  in  a  more  thorough  knowl- 
edge of  hydrated  oxide  of  iron  itself.  Our  knowledge  of  the  material  and 
the  chemical  and  physical  conditions  affecting  its  performance  is  incom- 
plete. Research  work  to  bring  forth  this  knowledge  is  needed.  Standard 
testing  methods  by  which  the  value  of  a  particular  material  can  be  quickly 
and  fairly  judged  are  also  needed.  Until  such  studies  are  made,  little 
radical  advancement  in  the  art  of  oxide  purification  is  to  be  looked  .for. 


Conclusion  63 

A  cooperation  for  carrying  on  this  work  has  been  entered  into  by  the 
American  Gas  Association  and  the  agencies  of  the  Illinois  Cooperative 
Mining  Investigations  and  it  is  hoped  that  research  work  now  in  progress 
will  solve  some  of  the  problems  of  oxide  purification. 


64  Gas  Purification  in  Medium  Size  Gas  Plants 

APPENDIX  A 

The  Steere  Engineering  Company1  formula  for  gas  purifiers:2 
n=3000  X  (D+C)  X  A 
S 

or, 
GXS 


3000  X  (D+C) 
Where  G=maximum  amount  in  cubic  feet  of  gas,  corrected  to  60°F., 
to  be  purified  per  hour. 

S=factor  for  grains  H2S  per  100  cu.  ft.  of  unpurified  gas,  as  given 
in  table  below. 

D=total  depth  of  oxide  through  which  the  gas  passes  consecutively 
in  the  purifier  set,  and  is  obtained  by  multiplying  the  depth  of  such  oxide 
per  box  by  the  number  of  boxes  in  series  in  the  set.  Where  a  single  catch 
box  is  used  for  two  or  more  sets,  disregard  the  catch  box  in  obtaining  the 
factor  "D". 


Note: — Duplex  boxes,  with  two  layers  of  oxide  each  and  divided  flow  of  gas,  whereby  half  the  gas 
passes  through  each  layer,  present  the  combined  area  of  the  two  layers  but  the  depth  of  one  layer  only 
to  the  passage  of  the  gas.  Therefore  the  area  "A"  of  a  duplex  box  is  the  sum  of  the  areas  of  the  two 
layers  of  oxide  or  double  the  cross-section  of  the  box.  The  depth  "D  '  is  the  depth  of  one  layer  of  oxide 
per  box  multiplied  by  the  number  of  boxes,  in  series,  in  the  set. 

A=cross-sectional  area  in  square  feet  of  the  oxide  through  which  the 
gas  passes'  on  its  way  through  any  one  box,  in  series,  of  a  set. 

C=f actor,  4  for  two-box,  8  for  three-box,  and  10  for  four-box  series, 
respectively. 

Where  a  single  catch  box  is  used  for  two  two-box  sets,  use  factor 
C=6. 

3000=assembling  constant. 
Value  of  S  is: 

Grains  H2S  per  100  cu.  ft.  unpurified  gas  Factor 

1000  or  more  720 

900  700 

800  675 

700  640 

600  600 

500  560 

400  525 

300  500 

200  or  less  480 


'For  detailed  explanation  see  Gas  Age,  vol.  43,  pp.  227,  1919,  or  Steere  Eng.  Co.,  Bulletin  No.  37, 
1919. 

2Reprinted  by  permission  of  the  Steere  Engineering  Company. 


Appendix  65 

APPENDIX  B 

Sample  record  forms  and  computations  for  keeping  account  of  oxide 
performance  and  the  status  of  various  oxide  batches  in  use: 

The  following  record  forms  are  suggested  only  as  embodying  most 
of  the  essential  features  of  a  record  which  would  give  the  gas  operator  a 
continuous  knowledge  of  the  performance  of  his  purifiers.  The  absorp- 
tion record  is  made  out  for  a  3-box  series  but  could  of  course  be  modified 
to  suit  the  conditions  in  any  given  plant.  The  actual  figures  given  are  not 
meant  to  indicate  just  what  absorptions  would  be  obtained  by  each  box; 
they  are  illustrative  only.  The  remarks  column  could  be  enlarged  to  give 
ample  space  for  other  notes,  such  as  when  the  direction  of  gas  flow  in  a 
particular  box  was  reversed.  In  the  case  illustrated  the  clean  box  is  pur- 
posely put  first  and  the  rotation  is  backward,  the  successive  sequences  being 
A  —  B  —  C,  C  —  A  —  B,  A  —  B  —  C.  This  method  might 
be  applicable  to  any  order  desired.  The  time  between  changes  of  sequence 
in  the  illustrated  form  has  no  significance.  In  some  cases  several  weeks 
might  elapse  between  changes  in  sequence  and  months  between  refillings 
of  a  box.  As  stated,  the  illustration  only  suggests  a  form  which  the 
operator  can  adapt  to  his  own  needs. 


66 


Gas  Purification  in  Medium  Size  Gas  Plants 


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Appendix  B 


67 


Where  a  box  is  emptied  during  the  month,  the  total  absorption  by 

that  batch  during  the  month  up  to  the  time  of  emptying  could  well  be 

entered  in  the  column,  preferably  in  red  ink.     Likewise  the  number  of  the 

batch  substituted   for  the  batch   removed  should  be  entered  in  the  same 

column  ahead  of  the  daily  entries.     The  total  absorption  by  each  batch 

while  in  a  box  is  entered  in  the  batch  record,  a  suggested  form  of  which 

follows  : 

Batch  Record 

Batch  No /     Kind  of  oxide Natural     Bought  from 

Date  purchased 1-3-20     Wt.  oxide  per  bu 25     Price  per  bu.  (on  ton) . . 

No.  bushels  in  bat  h 2500     Wt.  new  sponge  per  bu.  (air  dried) 35 


$20  Ion 


Date  put  into 
purifier 

Box 

No. 

Date  taken  out  of 
purifier 

Pounds  H2S 
absorbed 

Equivalent 
lbs.  sulphur 

Lbs.  sulphur 
per  bu.  oxide 

Remarks 

1/12/20 

A 
C 

/, 

B 
C 

7/6/20 

2/15/21 

8/17/21 

2/25/22 

6/20/22 

23500 
23000 
20000 
18000 
8000 

22100 
21600 
18820 
16900 
7540 

8.85 
8.65 
7.54 
6.76 
3.01 

8/1/20 

3/16/21 

41.0%  sulphur 

9/30/21 

3/15/22 

c;  rded 

Totals 

92500 

86960 

34.81 

%  sulphur  in  spent  oxide 


.5      %  tar  in  spent  oxide. 


.2.0 


%  cyanides. 


Note: --If  laboratory  facilities  are  available,  the  percentage  of  sulphur  in  the  oxide  could  be 
checked  each  time  before  the  batch  was  returned  to  use.  This  would  give  mere  accurate  total  results 
than  the  sum  of  the  Tutwiler  tests,  though  the  latter  are  very  useful  in  enabling  the  operator  to  know 
at  all  times  the  condition  of  his  purifying  material.  The  sulphur  equivalent  to  a  given  weight  of  H^S 
is  found  by  multiplying  the  latter  by  16  and  dividing  the  product  by  17.  The  batch  throughout  its 
usefulness  should  be  accompanied  by  a  batch  number,  prhferably  of  metal,  which  is  laid  on  the  box 
cover  or  otherwise  significantly  placed  while  the  batch  is  in  the  box  and  is  stuck  into  or  laid  upon  the 
batch  while  the  latter  is  revivifying  out  of  doors.  The  use  of  such  a  number  will  avoid  mistakes  as  to 
the  identity  of  a  given  batch. 

The  above  forms  could  of  course  be  printed  in  book  form  or  made  up 
as  desired.  The  absorption  record  would  require  a  page  per  month,  while 
the  batch  record  might  well  occupy  a  page  per*batch.  The  number  of 
entries  per  batch  would  depend  upon  the  practice  in  a  given  plant. 
Ordinarily  one  would  not  expect  to  handle  a  batch  more  than  four 
or  five  times,  perhaps  less,  but  there  are  conditions  which  make  many 
more  handlings  necessary.  Any  space  remaining  on  the  page  after 
the  entries  might  well  be  devoted  to  an  extension  of  the  remarks  giving 
some  information  relative  to  the  operating  methods  employed,  rapidity  of 
the  material  as  to  fouling  and  revivification,  presence  or  absence  of  tar, 
fineness  of  the  material  before  and  after  use,  caking,  etc.  Also,  if  any 
laboratory  tests  of  the  material  had  been  made  prior  to  or  during  use,  the 
agreement  of  these  tests  with  practical  results  observed  might  eventually 
be  valuable. 


68  Gas  Purification  in  Medium  Size  Gas  Plants 

APPENDIX  C 

Determination  of  the  further  usefulness  of  a  given  batch  of  oxide, 
according  to  the  formula1  of  Fulweiler  and  Kunberger:2 
Average  cost  of  purification  ==A^  per  M  cu.  ft.  of  gas 

Cost  of  new  oxide  =B^  per  bushel 

Residual  value  of  old  oxide  =C$  per  bushel 

Cost  of  removing,  revivifying,  and  replacing      =D<j;  per  bushel 
Gas  already  purified  by  one  bushel  ==E  thousands  of  cu.  ft. 

Number  of  times  oxide  has  already  been  used=F  times 
Average  H2S  content  per  M  cu.  ft.  =G  pounds 

Weight  of  one  bushel  of  oxide  =H  pounds 

Per  cent  of  H2S  removed  by  test  =1  per  cent 

Capital  charge  per  M  cu.  ft.  =JY'  per  M 

Then 

(B-C)  +  (FXD) 
A=  E :     +J 

But  J  is  a  constant   (practically  for  such  a  problem)   that  does  not 
affect  this  calculation,  so  we  may  write  it : 

n         ■                ,i     «      (B-C)  +  (FXD) 
Operating  cost=  ( A— J  )  = ^ 

The  question  then,  whether  or  not  it  will  pay  to  use  a  batch  again 
with  any  given  absorption  test  I,  depends  upon  whether 

(B-C)  +  (FXQD] 

E+    G 
is  equal  to;  greater  than;  or  less  than  the  average  (A — J).     For  the  sake 
of  a  numerical  example,  let  us  assume  that 
£A-J)=1.0^ 
B=35.0^ 
C=0 
D=5«5 

E=40.  (M  cu.  ft.  of  gas) 
F=4  times 
G=0.25  lbs. 
H=50  lbs. 
1=5% 
Then 

,A  (354)1(5X5) 

40+<^ 

'  Used  by  permission  of  W.  H.  Fulweiler. 

2 Some  of  the  phisical  characteristics  of  ferric  oxide:      Proc.  American  Gas  Institute,  1913,  Vol  • 
8,  Pt.  I,  p.  476. 


Appendix  C  69 

so  that  using  this  batch  would  increase  the  average  cost  considerably.  To 
get  the  minimum  economical  percentage  that  would  just  equal  the  average 
cost,  we  have — 

G  (  i 

I=    (A-J)XH        {   (B-C)  +  [(F+1)XD]-[(A-J)XE] 
Substituting  the  assumed  values  we  obtain — 

I=s        (1X50) ^   (35-0)  +  [(4+l)X5]-(lX^0) 

I=.10  or  10% 

In  applying  the  above  formula  part  of  the  information  is  known  and 

the  remainder  may  be  obtained  by  test.     For  example,  the  monthly  cost 

statement,  if  carefully  prepared  and  correct,  will  give  the  value  (A — J), 

viz.,  the  purifying  operating  cost   (includes  purifying  labor  and  purifying 

supplies  per  M).     Invoices  for  purifying  material  purchased  plus  freight 

and  handling  into  storage  give  the  value  B  when  figured  per  bushel.     The 

value  of  C  will  be  0  in  many  cases,  but  if  the  oxide  is  sold  or  used  in  such 

a  way  that  a  money  value  can  be  assigned  to  it,  then  the  value  per  bushel 

can  be  readily  obtained.    D  may  be  estimated  by  keeping  careful  account  of 

the  cost  of  emptying  a  box,  handling  the  oxide  during  revivification,  and 

putting  back  in  the  box,  and  dividing  by  the  number  of  bushels  so  handled. 

The  value  of  E  is  not  so  definite,  since  a  particular  batch  may  be  credited 

with  some  work  done  by  other  batches.     If  the  plan  for  keeping  account  of 

oxide  performance  suggested  in  the  text  of  this  paper  and  illustrated  in 

Appendix   B   is   followed,   no  difficulty  should   be  experienced,   since   the 

average  H2S  content  of  the  gas  (in  pounds  sulphur  per  M)   as  recorded, 

divided  into  the  pounds  of  sulphur  absorbed  per  bushel  by  the  oxide,  will 

give  the  thousands  of  cubic  feet  of  gas  already  purified.     Or,  if  an  analysis 

could  be  made  for  sulphur  content  of  the  oxide,  the  number  of  cubic  feet 

of  gas  of  average  H2S  content  corresponding  to  the  sulphur  per  bushel 

could  be  readily  computed.     For  example,  assume  that  the  air-dried  oxide 

averaged  30  per  cent  sulphur,  and  that  oxide  weighed  50  pounds  per  bushel, 

the  sulphur  per  bushel  would  be  .30X50=15  pounds. 

15  pounds=  105,000  grains  of  sulphur,  or 

105,000X34 
^ =112,000  grains  H2S. 

If  the  average  H2S  content  of  the  gas  is  100  grains  H2S  per  1  cu.  ft., 
then  this  gas  purified  by  the  oxide  per  bushel  is 

112,000 
j—  =  112,000  cu.  ft. 


70 


Gas  Purification  in  Medium  Size  Gas  Plants 


The  number  of  times  the  batch  in  question  has  been  used,  F,  can  also 
be  quickly  ascertained  from  the  batch  record  as  illustrated  in  Appendix 
B.  G.,  the  average  H2S  content  per  M  cu.  ft.  can  readily  be  obtained  from 
the  record  referred  to  over  any  period  of  time.  The  average  content  in 
grains  per  100  cu.  ft.  is  multiplied  by  10  and  the  product  divided  by  7,000 
to  convert  grains  to  pounds.  The  weight  per  bushel  of  oxide,  H,  can 
probably  be  obtained  with  sufficient  accuracy  by  spreading  out  and  air- 
drying  a  bushel  of  oxide  made  up  from  samples  taken  from  all  parts  of  the 
batch  and  then  weighing  on  a  good  scale.  The  value  of  I  has  to  be 
obtained  by  a  laboratory  test.  The  apparatus  employed  (aside  from  a 
chemical  balance,  which  is  essential)  is  shown  in  the  accompanying  sketch, 
Figure  5. 


HCl  or  H2SO4 


B 


Iron  Sulphide 


Calcium  Chloride 


Figure  5 — Kunberger  apparatus  for  testing  oxides  for  gas  purification.  A  is  a 
Kipp  gas  generator.  B  is  the  stop  cock  controlling  the  generation  and  flow  of  gas. 
C  is  the  drying  tube.  D  is  the  tube  containing  the  oxide  under  test  and  calcium 
chloride  for  absorbing  the  water  liberated. 

In  the  sketch,  A  is  Kipp  gas  generator  made  of  glass  in  which  H2S 
is  generated  by  the  action  of  hydrochloric  acid  or  dilute  sulphuric  acid  on 
ferrous  sulphide.  The  generation  and  flow  of  gas  is  controlled  by  the  stop- 
cock B.  The  U-tube  C  contains  granulated  fused  calcium  chloride  to  dry 
the  H2S.  The  calcium  chloride  tube,  D,  contains  in  the  straight  part  of 
the  tube,  5  grains  of  the  oxide  to  be  tested,  mixed  with  about  2  grains  of 
coarse  sifted  sawdust,  followed  by  granulated  fused  calcium  chloride  in  the 
bulb  of  the  tube.  Little  wads  of  glass  wool  are  placed  in  the  ends  of  this 
tube  and  between  the  oxide  and  the  calcium  chloride  to  retain  the  materials 
in  their  proper  places  and  prevent  any  from  dropping  out  while  weighing. 
Where  sponge,  viz.,  mixed  oxide,  is  to  be  tested,  no  sawdust  is  mixed  with 


Appendix  C  71 

the  samples.    A  coarsely  ground  sample  of  the  sponge  is  used.     Care  must 
be  taken  to  get  a  truly  representative  sample. 

After  the  tubes  are  filled,  D  is  weighed  on  a  chemical  balance.  Then 
it  is  connected  to  the  apparatus,  as  shown,  and  dried.  H2S  is  passed 
through  it  slowly  for. one  hour.  During  the  reaction  H2S  is  decomposed 
by  the  oxide  and  water  is  formed.  The  water  formed  is  retained  by  the 
calcium  chloride  in  the  bulb  of  the  tube  therefore,  the  gain  in  weight  of  the 
tube  during  the  time  gas  is  passing  through  the  tube  is  the  weight  of  the 
tube  during  the  time  gas  is  passing  through  the  tube  is  the  weight  of  the 
H2S  absorbed,  and  the  percentage  is  computed  by  dividing  this  gain  in 
weight  by  the  weight  of  the  sample  taken  for  test. 


PUBLICATIONS  OF 

ILLINOIS  MINING  INVESTIGATIONS.1 


ILLINOIS    STATE    GEOLOGICAL    SURVEY    DIVISION 
URBANA,  ILLINOIS 


Bulletin 
Bulletin 
Bulletin 
Bulletin 
Bulletin 

1. 

3. 
10. 
11. 
14. 

Bulletin 
Bulletin 
Bulletin 

15. 
16. 
17. 

Bulletin 

IS. 

Bulletin 
Bulletin 

20, 
21. 

Bulletin 

22. 

Bulletin 

23. 

Bulletin 

2fh 

Bulletin 

25. 

Preliminary  report  on  organization  and  method  of  investigations,  1913. 

Chemical  study  of  Illinois  coals,  by  S.  W.  Parr,  1916. 

Coal  resources  of  District  I  (Longwall),  by  G.  H.  Cady,  1915. 

Coal  resources  of  District  VII,  by  Fred  H.  Kay,  1915. 

Coal  resources  of  District  VIII   (Danville),  by  Fred  H.  Kay  and  K.  D. 

White,  1915. 
Coal  resources  of  District  VI,  by  G.  H.  Cady,  1916. 
Coal  resources  of  District  II  (Jackson  Co.),  by  G.  H.  Cady,  1917. 
Surface  subsidence  in  Illinois  resulting  from  coal  mining,   by  Lewis  E. 

Young,  1916. 
Tests  on  clay  materials  available  in  Illinois  coal  mines,   by  R.  T.  Stull 

and  R.  K.   Hursh,   1917. 
Carbonization  of  Illinois  coals  in  inclined  gas  retorts,  by  F.  K.  Ovitz,  1918. 
The   manufacture   of   retort   coal-gas    in    the    central    states,    using    loto- 

sulphur  coal  from  Illinois,  Indiana,  and  icedtern  Kentucky,  by  W.  A. 

Dunkley  and  W.  W.  Odell,  1918. 
Water-gas  manufacture  with  central  district  bituminous  coals  as  genera- 
tor fuel,  by  W.  W.  Odell  and  W.  A.  Dunkley,  1918. 
Mines    producing    loic-sulphur    coal   in    the   central    district,    by    G.    H. 

Cady,  1919. 
Water-gas   operating   methods    with    central   district    bituminous   coals   as 

generator  fuel.     A    summary  of  experiments   on    a   commercial  scale, 

by  W.  A.  Dunlcley  and  W.  W.  Odell.  1919. 
Gas   purification  in   the  medium-size   gas   plants    of   Illinois,   by   W.   A. 

Dunkley  and  C.  E.  Barnes,  19<!<>. 


Bulletin 

2. 

Bulletin 

4. 

Bulletin 

r>. 

Bulletin 

6. 

Bulletin 

7. 

Bulletin 

8. 

Bulletin 

9. 

Bulletin 

12. 

Bulletin 

13. 

Bulletin 

91. 

Bulletin 

100. 

ENGINEERING  EXPERIMENT  STATION 

URRAXA.    ILLINOIS 

Coal  mining  practice  in  District  VIII  (Danville),  by  S.  O.  Andros,  1913. 

Coal  mining  practice  in  District  VII,  by  S.  O.  Andros,  1914. 

Coal  mining  practice  in  District  1    iLongwall),  by  S.  O.  Andros,  1914. 

Coal  mining  practice  in  District  V,  by  S.  O.  Andros,  1914. 

Coal  mining  practice  in  District  II.  by  S.  O.  Andros,   1914, 

Coal  mining  practice  in  District  VI.  by  s.  o.  Andros,   1914. 

Coal  mining  practice  in  District  III,  by  S.  O.  Andros,  101."). 

Coal  mining  practice  in  District  IV.  by  S.  ().  Andros.  1P1.">. 

Coal  mining  in    Illinois,   by   S.   O.   Andros,    1915.      (Complete  resume  of 

all  the  district  reports.) 
Subsidence  resulting  from  mining,  by  L.  B.  Young  and  II.  II.  Stock.   1916, 
Percentage   of   extraction    of   bituminous   coal   witb    special    reference   to 

Illinois   conditions,   by  (\   M.   Young.    1017. 


U.  S.  BUREAU  (IF  MINES 
WASHINGTON.    I).   C. 

Bulletin     72.     Occurrence  of  explosive  gases  in  coal  mini's,  by   X.  II.  Darton,   1!>1.~>. 

Bulletin      83.      The  humidity  of  mine  air,  by   R.   Y.  Williams,  1014. 

Bulletin     99.     Mine  ventilation  stoppings,  by  R.  Y.  Williams,  1915. 

Bulletin  102.     The  inflammabilitv   of  Illinois  coal  dusts,   by  J.  K.  Clement  and  L.  A. 

Scholl,  Jr.,  1916. 
Bulletin  137.     Use   of  permissible   explosives   in   the  coal  mines  of   Illinois,   by   J.    R. 

Fleming  and  J.  W.  Koster.  1917. 
Bulletin  138.     Coking  of  Illinois  coals,  bit  F.   K.  Oritz,  1917. 
Technical     Paper     190.      Mfithane     accumulations     from     interrupted     ventilation,     with 

special     reference    to    coal    mines    in     Illinois    and     Indiana,    by     II.     f. 

Smith  and   Robert  J.   Hamon,   1918. 
1  Bulletins  listed  in  italics  apply  directly  to  the  problem  of  use  of  central  district 
bituminous  coals  in  place  of  eastern  coal  and  coke. 


